JP3081615B2 - Liquid crystal electro-optical device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a liquid crystal electro-optical device.

[Prior Art] As shown in FIG. 8, a conventional liquid crystal electro-optical device comprises a liquid crystal cell 1 and polarizing plates 3, 4 arranged on both sides of the liquid crystal cell.
Consists of In particular, in the case of the transmission type, the backlight 5 is provided outside the polarizing plate 4, and in the case of the reflection type, the backlight 5 is similarly formed.
Was provided.

[Problems to be solved by the invention] In recent years, with the increase in the number of pixels required for the display,
The density of circuit elements formed on a substrate of a liquid crystal electro-optical device is rapidly increasing, which causes a decrease in the amount of transmitted light. For example, the aperture ratio defined by the displayable area occupied in the pixel area is about 40% in a current liquid crystal cell used for a projection television or the like, but if the number of pixels corresponding to a high definition is increased in the future, 10%. %. For the purpose of improving this, the adoption of a backlight and an increase in the amount of the backlight, or the improvement of the structure and design of circuit elements have been performed, but the effect has been insufficient.

The present invention solves such a problem, and an object of the present invention is to improve the aperture ratio to about 90% at a stroke by introducing a new reflective liquid crystal mode, and to increase the density of circuit elements. An object of the present invention is to provide a liquid crystal electro-optical device that does not cause a decrease in the amount of transmitted light.

[Means for Solving the Problems] In a liquid crystal electro-optical device according to the present invention, a liquid crystal having a twist orientation of about 90 degrees is sandwiched between a pair of substrates. A liquid crystal cell in which a reflective film is formed on a liquid crystal side surface, and light of the first polarization axis component is reflected on the other substrate side, which is the light incident side of the liquid crystal cell, and is orthogonal to the first polarization axis component. A selective reflection member transmitting the light of the second polarization axis component is disposed, and the rubbing direction of the rubbing applied to the other substrate and the rubbing direction of the light emitted from the selective reflection member cross each other. The other substrate is disposed, light emitted from the selective reflection member is reflected by the reflection film, and after reflection, is incident on the selective reflection member. The liquid crystal electro-optical device does not apply a voltage to the liquid crystal cell. State in a high reflectance state, And having electro-optical properties as a low reflectance state in the voltage applied state of the crystal cell.

[Operation] The selective reflection member is formed, for example, by forming a multilayer thin film 22 made of a metal or a metal oxide on the slopes of two prism-shaped light transmitting bodies 21 as shown in FIG. It was made. For the slope, as shown in FIG.
When the light 51 is incident at an angle of 45 degrees, the P-polarized light component 52 (polarized light whose vibration plane is perpendicular to the inclined surface) goes straight out without being affected by the selective reflection member, and is emitted. 53 (polarized light whose light oscillation direction is parallel to the slope) is reflected in a direction at 90 degrees to the incident direction. Such a selective reflection member is particularly called a polarizing beam splitter (hereinafter abbreviated as PBS).

PBS exhibits a similar effect to a conventional polarizing film in that it selectively transmits polarized light. However, instead of having two polarizers, a polarizer and an analyzer, on both sides of the liquid crystal cell as in a conventional liquid crystal electro-optical device, one PBS serves as both a polarizer and an analyzer. It is necessary to use a new liquid crystal mode different from the above.

Conventionally, for such an optical system, it has been proposed to use a homeotropically aligned n-type nematic liquid crystal. The display principle will be described with reference to FIG. Homeotropically aligned liquid crystals do not have birefringence in the cell normal direction. Therefore, the S-polarized light 53 incident from the direction A in the drawing and bent at a right angle by the PBS exits the cell in the state of the S-polarized light 56 as it is. The S-polarized light 56 is reflected at right angles by the PBS again and exits in the A direction, and is not observed from the B direction. On the other hand, a voltage is applied to the liquid crystal cell to tilt the liquid crystal molecules,
When the retardation is set to λ / 4 (a quarter of the wavelength of the light, but the actual retardation is λ / 2 because of the reflection type), the incident S-polarized light 53 becomes P-polarized light.
It is emitted in the state of 55. The P-polarized light 55 travels straight and is emitted in the B direction without being affected by the PBS. Thus, the homeotropic cell can be turned on / off with high contrast in the reflection mode combined with PBS. However, the homeotropic liquid crystal cell is difficult to align and cannot be reliable, and the n-type liquid crystal material is still under development and has not obtained sufficient characteristics. Not practical.

Conversely, it is also possible to use a homogeneously aligned p-type nematic liquid crystal without twist. In this case, the light becomes bright when no voltage is applied, and becomes dark when the voltage is applied and the liquid crystal molecules stand sufficiently. Since the homogeneous alignment is different from the homeotropic alignment and has been widely used in the past, all the techniques are at a practical level. However,
As the retardation is lost, a very high voltage is required to make the liquid crystal molecules stand, and a high contrast cannot be obtained with a normal voltage as in a homeotropic alignment cell. This is because, as shown in FIG. 6, the liquid crystal molecules in the vicinity of the substrate of the liquid crystal cell are difficult to stand due to the surface regulating force of the alignment film.

The liquid crystal electro-optical device of the present invention is characterized in that the twist angle is about 90 degrees while being homogeneously aligned. By setting the twist angle to about 90 degrees, the liquid crystal molecules near the upper and lower substrates that are orthogonal to each other optically compensate each other, and the retardation is lost at a relatively low voltage, so that a dark state is obtained. On the other hand, reflectance (brightness) when no voltage is applied
For, it is necessary to optimize the cell conditions. FIG. 7 shows the change in the reflectance when the value of the angle θ between the polarization direction of the incident light and the rubbing direction of the upper substrate and the value of the retardation Δn × d of the liquid crystal cell are changed. According to this figure, the reflectance is θ = 0 to 40 degrees and Δn × d = 0.15 μm to 0.
35 μm, θ = 50 degrees to 70 degrees, Δn × d = 0.60 μm to 0.75
It can be seen that a high value is obtained under the two conditions of μm. As described above, a dark state is always obtained when a voltage is applied, so that under these conditions, ON / OFF with high contrast is possible.

EXAMPLES Hereinafter, the present invention will be described in detail with reference to examples.

Embodiment 1 FIG. 1 is a sectional view of a liquid crystal electro-optical device according to the present invention. In the figure, 1 is a liquid crystal cell, and 2 is PBS. Further, 11 is an upper substrate, 12 is a lower substrate, 13 is a transparent electrode, 14 is a reflective film also serving as a pixel electrode, 15 is a thin film transistor (hereinafter referred to as TFT), 16 is an insulating film, and 17 is a liquid crystal. The liquid crystal used was ZLI-2359 (Δn = 0.0512) manufactured by Merck and had a cell gap of
The liquid crystal cell of 4.9 μm was homogeneously aligned at a twist angle of 90 degrees left. The retardation is 0.25 μm. A nickel vapor deposition film was used for the reflection film, and a polyimide resin was used for the insulation film. TFT that caused a decrease in aperture ratio
Since the reflective film covers almost the entire surface, the aperture ratio is improved to 92%.

FIG. 2 is a diagram showing the relationship between each axis of the liquid crystal electro-optical device of the present invention as viewed from the light incident direction. 31 is the polarization direction of the incident light that has passed through the PBS, 32 is the rubbing direction of the upper substrate,
33 is the rubbing direction of the lower substrate. Reference numeral 34 denotes an angle θ formed by 31 and 32 (a counterclockwise value is a positive value), and reference numeral 35 denotes a twist angle of the liquid crystal. In the first embodiment, θ is set to 20 degrees.

FIG. 3 is a diagram showing the electro-optical characteristics of the liquid crystal electro-optical device manufactured under the above conditions. The reflectance is 76%,
Nearly comparable to traditional twisted nematic mode, 92
In addition to the high aperture ratio of 10%, bright display became possible. The display contrast is up to 1:85.

(Example 2) In Example 1, except that the twist angle of the liquid crystal was 95 degrees, the retardation was 0.20 μm, and the angle θ was 105 degrees,
It was the same as in Example 1. Increasing the twist angle of the liquid crystal allows driving at a lower voltage, but reduces the reflectivity to 70%. The angle θ is 90
Even if an integer multiple of degrees is added, the characteristics do not change.
The degree is the same as θ = 15 degrees.

(Example 3) The procedure of Example 1 was repeated, except that the retardation was 0.70 μm and the angle θ was 60 degrees. This is another high reflectivity condition shown in FIG. Although it can be driven at a lower voltage and improves the sharpness of the threshold voltage as compared with the first embodiment, it has the disadvantage that the coloring of the display increases.

Example 4 In this example, two polarizing plates were used instead of using PBS. FIG. 4 is a sectional view of the liquid crystal electro-optical device according to the fourth embodiment. Light is about 10 from the normal direction of the liquid crystal cell.
The light is incident from a direction inclined at an angle of 10 degrees, and is emitted in a direction also inclined by about 10 degrees. The polarizer 3 and the analyzer 4 were placed on the optical paths of the incident light 51 and the output light 54, respectively, to substitute for PBS. The conditions of the liquid crystal cell were set the same as in Example 1.

Since the polarizing plate has a higher degree of polarization than PBS, a high display contrast of 1: 220 could be obtained.

[Effects of the Invention] As described above, according to the present invention, by introducing a new reflective liquid crystal mode, there is provided a liquid crystal electro-optical device which does not cause a decrease in the amount of transmitted light even when the density of circuit elements increases. be able to.

[Brief description of the drawings]

FIG. 1 is a sectional view of a liquid crystal electro-optical device according to the present invention. FIG. 2 is a diagram showing the relationship between each axis of the liquid crystal electro-optical device of the present invention. FIG. 3 is a diagram illustrating the electro-optical characteristics of the liquid crystal electro-optical device according to the first embodiment of the present invention. FIG. 4 is a sectional view of a liquid crystal electro-optical device according to Embodiment 4 of the present invention. FIG. 5 is a diagram showing the action of PBS. FIG. 6 is a diagram schematically showing an arrangement of liquid crystal molecules when a voltage is applied to the liquid crystal cell. FIG. 7 is a diagram showing how the reflectivity of the liquid crystal electro-optical device of the present invention changes depending on Δn × d and the angle θ of the liquid crystal cell. FIG. 8 is a sectional view of a conventional liquid crystal electro-optical device. DESCRIPTION OF SYMBOLS 1 ... Liquid crystal cell 2 ... Selective reflection member (here, PBS) 3 ... Polarizing plate (polarizer) 4 ... Polarizing plate (analyzer) 5 ... Reflector or backlight 11 ... Upper substrate 12 ... Lower substrate 13 Transparent electrode 14 Reflective film also serving as pixel electrode 15 Circuit element (here TFT) 16 Insulating film 17 Liquid crystal 21 Prism 22 Multilayer thin film 31 PBS Polarization direction of incident light that has passed through 32: Rubbing direction of upper substrate 33: Rubbing direction of lower substrate 34: Angle θ between 32 and 31 35: Twist angle of liquid crystal 41: Wavelength of 450 nm (blue) Electro-optical properties of light 42 ... Electro-optical properties of light with a wavelength of 550 nm (green) 43 ... Electro-optical properties of light with a wavelength of 650 nm (red) 51 ... Incident light 52 ... P-polarized component of incident light 53 ... S-polarized light component of incident light 54... Outgoing light 55... P-polarized light component of outgoing light 56.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-1-149087 (JP, A) JP-A-63-175828 (JP, A) JP-A-61-177445 (JP, A) JP-A-61-177445 166522 (JP, A) JP-A-61-73988 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G02F 1/133 500 G02F 1/1335 520

Claims (1)

(57) [Claims] 1. A liquid crystal in which a liquid crystal twisted at about 90 degrees is sandwiched between a pair of substrates, and a reflective film is formed on a surface of one of the pair of substrates on the liquid crystal side. A cell and a selective reflection member that reflects light of the first polarization axis component on the other substrate side, which is the light incident side of the liquid crystal cell, and transmits light of the second polarization axis component orthogonal to the first polarization axis component The other substrate is arranged so that a polarization direction of light emitted from the selective reflection member and a rubbing direction of rubbing applied to the other substrate intersect, and the selective reflection member is provided. The light emitted from the liquid crystal is reflected by the reflection film, is incident on the selective reflection member after reflection, and the liquid crystal electro-optical device is in a high reflectance state when no voltage is applied to the liquid crystal cell, and the voltage of the liquid crystal cell is changed. Low reflectivity state A liquid crystal electro-optical device having the following electro-optical characteristics.