JPH03118513A - Liquid crystal display device - Google Patents

Abstract

PURPOSE:To offer the two-layered type liquid crystal display device with small variance in contrast by providing a 1st and a 2nd liquid crystal cell each having a couple of transparent substrates which differ in retardation value from each other. CONSTITUTION:A liquid crystal cell 1 for display has a couple of transparent substrates 23a and 23b and many picture element electrodes 50 made of ITO are arranged on the internal surface of one substrate 23b in matrix. A liquid crystal cell 2 for compensation has a couple of transparent substrates 22a and 22b. The liquid crystal cell 1 for compensation is provided with no electrode. The liquid crystal cell 1 for display and liquid crystal cell 2 for compensation contact each other across the substrates 23a and 22b. The retardation value of the liquid crystal cell 2 for compensation is set to the best value different from the retardation value of the liquid crystal cell 1 for display and thus the variance in contrast is reduced. Therefore, the liquid crystal display device with small variance in image quality is obtained.

Description

[Detailed description of the invention] (Industrial application field) The present invention relates to a two-layer liquid crystal display device.

(Prior Art) Liquid crystal display devices are used in a wide range of fields such as computer terminals, word processor displays, and television screens. Among the liquid crystal display devices used in these fields, there is a normally black type that blocks light when no voltage is applied, and a two-layer type that has two liquid crystal cells (Japanese Patent Publication No. 63-53528, Tokuko Sho 63-53
No. 529, JP-A No. 63-234225, etc.). A two-layer normally black type liquid crystal display device is characterized in that the light blocking effect when no voltage is applied does not depend on the wavelength of the light.

In such a liquid crystal display device, two liquid crystal cells in which liquid crystal molecules are oriented in twisted nematic orientation or super twisted nematic orientation are superimposed. One of the two liquid crystal cells is a display liquid crystal cell provided for performing a display operation. The other is a compensating liquid crystal cell for returning the light that has become elliptically polarized light due to optical rotation dispersion in the display liquid crystal cell to linearly polarized light. An electrode for applying a voltage to the liquid crystal layer is provided on the inner surface of each substrate of the display liquid crystal cell. In the two liquid crystal cells, the long axis of the liquid crystal molecules and the plane perpendicular to the direction of light propagation are almost parallel, and the liquid crystal molecules are oriented in a spiral while gradually twisting from one substrate to the other of the liquid crystal cells. are doing. Furthermore, the two liquid crystal cells are oriented in opposite helical directions. In a portion where two liquid crystal cells are close to each other, the alignment directions of liquid crystal molecules in each cell are arranged to be orthogonal to each other.

If a two-layer liquid crystal cell with opposite helical directions is used in this way, the light that becomes elliptically polarized light due to optical rotational dispersion in the display liquid crystal cell will be returned to linearly polarized light due to optical rotational dispersion in the compensation liquid crystal cell. . In this way, in a two-layer liquid crystal display device, the generated elliptically polarized light is returned to linearly polarized light, so the light blocking effect does not depend on the wavelength of the light, and the light blocking rate is 1%.
It is as follows.

(Problem to be Solved by the Invention) In a conventional two-layer liquid crystal display device, the values of water duration R in the two liquid crystal cells are set to be equal so that the light blocking effect does not depend on the wavelength of the light. . Watadasion R is a value expressed as R = d - Δn"cos2θ. Here, d is the gap of the liquid crystal cell (thickness of the liquid crystal layer), Δn is the refractive index anisotropy, and θ is the average of the liquid crystal molecules. This is the pretilt angle.Normally, the same liquid crystal material is used to fill the two liquid crystal cells.Therefore, the gaps between the two liquid crystal cells are set to be equal.

However, the gap of a liquid crystal cell manufactured in an actual manufacturing process is not completely constant and has some degree of variation. Due to such variations, the contrast of the obtained liquid crystal display device also varies. Hung trust is one of the major factors that determines image quality. Therefore, a display device with low contrast will reduce yield. In conventional liquid crystal display devices, it is impossible to completely keep variations in the gap between liquid crystal cells constant, and therefore it is impossible to avoid variations in image quality.

The present invention solves these problems, and an object of the present invention is to provide a two-layer liquid crystal display device with small variations in contrast.

(Means for Solving the Problems) The liquid crystal display device of the present invention includes a first liquid crystal cell and a second liquid crystal cell each having a pair of transparent substrates, and a liquid crystal cell provided on the inner surface of the first liquid crystal cell. A liquid crystal display device comprising an electrode, and a first liquid crystal layer and a second liquid crystal layer that are respectively sealed in the first liquid crystal cell and the second liquid crystal cell and twisted in opposite spiral directions. The water duration value of the second liquid crystal cell is different from the water dage value of the first liquid crystal cell, thereby achieving the above object.

To set the water duration value of the second liquid crystal cell to a value different from that of the first liquid crystal cell, (1) set the gap of the second liquid crystal cell to a value different from that of the first liquid crystal cell; (2) A liquid crystal material having a refractive index anisotropy different from that of the first liquid crystal layer is used for the second liquid crystal layer.

The change in the light transmittance T (%) of the display device when the value of the water dage R2 of the second liquid crystal cell is changed from a constant value 1 of the water dage R1 of the first liquid crystal cell is expressed as the fourth value.
As shown in the figure. In FIG. 4, R+=0. A case of a constant value of 477 is shown. As shown in FIG. 4, T has a minimum value when R2=R. When R2<R, the transmittance T rises relatively steeply.

Conversely, when R2>R, T is rising relatively slowly.

In the conventional liquid crystal display device described above, R2 is set to a value equal to R1, so R2 of the actual display device obtained is
will be distributed around R1. Among actual display devices having such a distribution of R2, R2<R,
In a display device, R2>R.

The value of the transmittance T tends to be larger than that of the display device. In a display device having such a large transmittance, the contrast will be reduced.

In the liquid crystal display device of the present invention, the value of R2 is set to R2 as described above.
It is not set to the same value as 1, but is set to a different value. That is, the value of R2 is R1 where T changes relatively slowly.
is set to a larger value R2*. By setting the value of R2 to a value larger than R1 in this way, the change in T with respect to the change in R2 becomes small. Therefore, variations in contrast of the resulting liquid crystal display device can be reduced.

(Example) The invention will now be described with reference to examples.

FIG. 1 shows a schematic cross-sectional view of one embodiment of the liquid crystal display device of the present invention. In this embodiment, two twisted nematic mode liquid crystal cells, namely a display liquid crystal cell 1 and a compensation liquid crystal cell 2, are provided.

This display device uses an active matrix driving method.The display liquid crystal cell 1 has a pair of transparent substrates 23a and 23b.
On the inner surface of one substrate 23b, a large number of picture element electrodes 50 made of oxide are arranged in a matrix. Each picture element electrode 50 is equipped with a thin film transistor (r
(referred to as TFTJ) 40 are connected to constitute an active matrix substrate 30. In Figure 1, for simplicity,
Only one picture element electrode 50 and only one TFT 40 are each schematically shown. Details of the planar configuration of the active matrix substrate 30 and the cross-sectional configuration of the TFT 40 will be described later. A counter electrode 51 made of ITO is formed on the entire surface of the substrate 23a. Alignment films 24 and 24 are provided on the TFT 40, the picture element electrode 50, and the counter electrode 51.

Similarly, the compensation liquid crystal cell 2 has a pair of transparent substrates 22a and 22b. The compensation liquid crystal cell 2 is not provided with any electrodes. Alignment films 24 and 24 are provided on the inner surfaces of the respective substrates 22a and 22b. The four alignment films 24 formed on the display liquid crystal cell 1 and the compensation liquid crystal cell 2 are formed to a thickness of 600 mm by offset printing polyimide resin, and then rubbed with a nylon thread fabric. ing.

The display liquid crystal cell 1 and the compensation liquid crystal cell 2 are in contact with each other through substrates 23a and 22b, respectively.

Polarizing plates 21a and 2 are provided on the outside of the substrates 22a and 23b.
1b is provided. These polarizing plates 21a and 21
b are arranged to form orthogonal Nicols.

between the substrates 22a and 22k), and between the substrates 23a and 23b
The liquid crystal layers 26 and 28 are placed between the sealing material 2 and the liquid crystal layers 26 and 28, respectively.
It is enclosed by 5. In both liquid crystal layers 26 and 28, the twist angle of liquid crystal molecules is 90°. However, the liquid crystal molecules in the liquid crystal layer 28 are oriented levorototally, and the liquid crystal molecules in the liquid crystal layer 28
The liquid crystal molecules are dextrorotated. That is, the spiral directions of the two liquid crystal cells 1 and 2 are set to be opposite to each other. Then, the alignment direction of liquid crystal molecules near the substrate 22b of the liquid crystal layer 26 and the substrate 23a of the liquid crystal layer 28 are determined.
The alignment directions of liquid crystal molecules in the vicinity of are arranged so as to be orthogonal to each other. The directions of the rubbing treatment of the four alignment films 24 enable such alignment of the liquid crystal molecules in the liquid crystal layer 26 and the liquid crystal layer 28.

FIG. 2 shows a plan view of the active matrix substrate 30 constituting the liquid crystal display device of FIG. 1. Picture element electrodes 50 are arranged in a matrix on the transparent substrate 23b.
Gate bus wiring 41 is formed parallel to one direction between 0 and 0. A source bus wiring 47 is provided perpendicularly to the gate bus wiring 41 . A TFT 40 is provided as a switching element near the intersection of the gate bus line 41 and the source bus line 47. The picture element electrode 50 is driven by the TFT 40.

The cross-sectional configuration of the TFT 40 along the mm line in FIG.
As shown in the figure. A gate bus wiring 41 is formed on the transparent substrate 23b, and a part of the gate bus wiring 41 functions as a gate electrode of the TFT 40. An anodic oxide film 42 is formed on the gate bus wiring 41, and a gate insulating film 43 is deposited on the entire surface of the anodic oxide film 42. A semiconductor layer 44 is formed on a portion of the gate bus wiring 41 that functions as a gate electrode, with the anodic oxide film 42 and gate insulating film 43 mentioned above interposed therebetween. A semiconductor layer protective film 45 is formed on the semiconductor layer 44 to protect the semiconductor layer 44 . Further, two contact layers 46 are formed on the semiconductor layer 44.
and 46 are provided, and a source electrode 52 and a drain electrode 48 are formed on the contact layers 46 and 46, respectively.

The source electrode 52 is connected to the source bus wiring 47, and the drain electrode 48 is connected to the picture element electrode 50. A protective film 49 is formed on the TFT 40 formed in this way,
An alignment film 24 is formed on the entire surface of the protective film 49.

The liquid crystal material used for the display liquid crystal cell 1 is a PCH (phenylcyclohexane) mixed liquid crystal.

The refractive index anisotropy of this liquid crystal material is 0.09.

Cholesteric nonanate was added to the liquid crystal layer 28 of the display liquid crystal cell 1 to impart levorotatory properties.

The cell gap d1 between the substrates 23a and 23b of the display liquid crystal cell 1 is set to about 5.3 μm by dispersing plastic spacers. Therefore, the retardation R+ of the display liquid crystal cell 1 is 0. 477 (CO5
20#1. 0).

The cell gap d2 between the substrates 22a and 22b of the compensation liquid crystal cell 2 is set to 4.5 μm. The liquid crystal material used for the compensation liquid crystal cell 2 is a PCH-based mixed liquid crystal similar to that of the display liquid crystal cell 1, but by changing the mixing ratio, the refractive index anisotropy value can be varied from 0.09 to 0.0. It was varied between 13. Further, CB-15 was added to the liquid crystal layer 26 of the compensation liquid crystal cell 2 as a dextrorotatory chiral substance. The refractive index anisotropy value of 0.09 to 0.13 corresponds to the retardation R2 of the compensation liquid crystal cell 2 of 0.405 to 0.585 (CO82θ#1.O).

The transmittance of the display device when the retardation R1 of the display liquid crystal cell 1 is fixed at 0.477 as described above and the retardation R2 of the compensation liquid crystal cell 2 is varied between 0.405 and 0.585. Figure 4 shows the change in T. As shown in FIG. 4, when R2 is equal to R1, that is, R2=0
．． The minimum value is 477. When R2 is smaller than R1, the transmittance T rises relatively steeply.

Conversely, if R2 is greater than R1, T is rising relatively slowly.

In conventional ordinary liquid crystal display devices, the dispersion of R2 is ±
It is 0.05. If the setting value of R2 is set to the same value as R1 as in the conventional display device, the transmittance T becomes 0.0 as determined from FIG. It has a fluctuation range of 3 to 2.5%. In this example, based on the results shown in FIG.
I set it to 95.

By setting R2 to 0.495, the variance of R2 ±
The fluctuation range of the transmittance T corresponding to 0.05 is 0°3 to 1.
It became 35%. In this way, in this embodiment, the range in which the transmittance fluctuates is about half that of the conventional display device described above.
reduced to

Note that the setting value of the retardation R2 of the compensation liquid crystal cell 2 is determined by taking into consideration the variation in the thickness of the two cells, the optical characteristics of the liquid crystal material used, and the like.

(Effects of the Invention) In the liquid crystal display device of the present invention, the retardation value of the compensation liquid crystal cell is set to an optimum value different from the retardation value of the display liquid crystal cell, so that variations in contrast are reduced. .

Therefore, according to the present invention, a liquid crystal display device with small variations in image quality can be provided, and it can contribute to improving the yield of display devices.

4. Don't worry! Nitto Figure 1 is a schematic cross-sectional view of one embodiment of the present invention, and Figure 2 is a schematic cross-sectional diagram of one embodiment of the present invention.
3 is a cross-sectional view taken along the line m-m in FIG. FIG. 3 is a diagram showing the relationship between values and transmittance of a display device.

1...Display liquid crystal cell, 2...Compensation liquid crystal cell, 2
1 a, 2 l b-” polarizing plate, 22 a, 2
2 b, 23 a.

23b...Transparent substrate, 24...Alignment film, 25...
-Sealing material, 26.28...Liquid crystal layer, 30...Active matrix substrate, 40...TFT, 41...
Gate bus wiring, 43... Gate insulating film, 44...
Semiconductor layer, 47... Source bus wiring, 50... Picture element electrode, 51... Counter electrode.

that's all

Claims (1)

[Claims] 1. A first liquid crystal cell and a second liquid crystal cell each having a pair of transparent substrates, an electrode provided on the inner surface of the first liquid crystal cell, and an electrode provided on the inner surface of the first liquid crystal cell and the second liquid crystal cell. A liquid crystal display device comprising a first liquid crystal layer and a second liquid crystal layer, each of which is encapsulated and twisted and oriented in opposite spiral directions, the retardation value of the second liquid crystal cell being equal to or greater than the second liquid crystal layer. A liquid crystal display device having a retardation value different from that of the liquid crystal cell in item 1.