Liquid crystal display device and display apparatus using same

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LIQUID CRYSTAL DISPLAY DEVICE AND
DISPLAY APPARATUS USING SAME

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to a display device using liquid crystals, especially to a liquid crystal display device and a display apparatus for being used in office information apparatus (OA apparatus) etc.

(2) Description of the Prior Art

Many kinds of liquid crystal display devices have been proposed and come to practical use as display devices for information display apparatus. At the pres- J5 ent, a type using nematic liquid crystals represented by TN mode (Twisted Nematic; Refer to JP-A-47-11737) and STN mode (Super Twisted Nematic; Refer to JPA-60-107020) is the main current and is widely used.

The TN mode and the STN mode have initial structural condition in which each of oriented direction of liquid crystal molecules is twisted approximately 90° or 260° in the cell. Incident light to the cell outgoes after being changed in polarized condition by the twisted structure of the liquid crystals and birefringence. When 25 electric field is applied to the liquid crystal layer, the twisted structure vanishes by rearrangement of the liquid crystal molecules in direction of the electric field, and the outgoes without any change in the polarized condition. By composing a structure in which a liquid 30 crystal cell is held between two linear polarizers, above described change (electro-optical effect) in optical property of the liquid crystal layer by application of the electrical field can be observed as change of outgoing light intensity. 35

With TN mode and STN mode, display contrast is obtained by the operation based on the above described principle.

The above described display device has such advantages as remarkably smaller consuming electric power 40 than CRT (Cathode Ray Tube) display and making it possible to realize thin display panel. Accordingly, the apparatus is used widely in office information apparatus such as personal computers and word processors etc.

However, the above described display apparatus is 45 substantially a type provided with polarizers, and the incident light is not utilized effectively. Actually, there are many cases of display in which a light source (a back light) is provided behind the liquid crystal display device for keeping brightness. A type provided with color 50 filter (Color liquid crystal display) reduces quantity of transmitted light farther, and consequently, it needs stronger light source. Electric power consumed by the light source is equivalent to the consuming electric power of the liquid crystal panel including driving cir- 55 cuits, therefore, such type is not suitable for display of portable type information apparatus of which electric power is supplied from batteries. That is, brightness and lowering of consuming electric power in the conventional display device are in the so-called trade-off rela- 60 tion, and development of bright (high light transmittal) display device which does not need the back light is earnestly desired.

Fluorescent back light is not preferable in aspect of eye fatigue when watching the display continuously, 65 and reflection type display is desired. Besides, the display device having high light transmittance contributes to reduction of light source size, extension of life, and

electric power saving of the whole apparatus even in the case of being used as a projection type display.

In response to the above described need, a liquid crystal display device without using polarizers is proposed. The White-Taylor type guest-host cell [Refer to Journal of Applied Physics (J. Appl. Phys.) Volume 45, page 4718-4723 (1974)] is one of such display devices. Chiral nematic phase (cholesteric phase) liquid crystal is mixed with dichroic dye, and has a structure of approximate parallel configuration to the base plane. By applying electric field, the configuration of the liquid crystal changes, and consequent orientation change of the dichroic dye changes light transmittance. Owing to the twisted structure caused by the chiral nematic phase, light absorption by the dye is performed effectively, and theoretically high display contrast can be obtained without polarizers.

In the above described case, spiral pitch of the chiral nematic liquid crystal is necessarily same order of wave length of the light for realizing of high contrast. But, if the spiral pitch is shortened to that order, many disclination lines are generated and display quality becomes worse. At the same time, hysteresis occurs and response to the electric field becomes extremely slow. Accordingly, it is less practical in comparison with the TN mode and the STN mode.

Another representative display device without using polarizers is a display device called PDLC (Polymer Dispersed Liquid Crystal: Refer to JP-A-58501631(1983)).

The above described device consists of polymer matrix in which nematic liquid crystal having positive dielectric anisotropy is dispersed as particles having diameter of a several micrometers. When refractive indexes of the liquid crystal to ordinary light and extraordinary light are represented by n0 and ne respectively, and refractive index of the polymer is represented by tip, combination of liquid crystal and polymer material are so selected as to be n^n^ne. In the initial condition, the liquid crystals form a warped configuration structure in the particles, furthermore, difference of refractive index between most of the liquid crystal particles and the polymer matrix is induced by fluctuation in oriented direction among each of particles, consequently, the device scatters light as if it were frosted glass. When sufficient voltage is applied to the device, rearrangement of the liquid crystal molecules in the particles is induced and refractive indexes of the liquid crystal and polymer matrix to perpendicularly incident light become equal value. As the result, refraction and reflection at the boundary of the liquid crystal and polymer are almost disappeared, and the device changes to transparent state. The incident light is not necessarily linear polarized light.

As the displays based on the above described theory, polarizers are not necessary, and the incident light can be utilized effectively for obtaining bright display. But, in order to obtain sufficient display contrast, thick film having thickness of a several tens micrometers is necessary, consequently, its operating voltage becomes a several tens volts. Furthermore, as the device is dispersion type, it is preferable for the projection type display but not for the direct view type display such as OA apparatus etc.

Although a reflection type display, in which dichroic dye is mixed into nematic liquid crystal, contrived reflectors or cell construction is proposed (Refer to JP-A59-178429 (1984) and JP-A-59-178428 (1984)), the im3

provement is not substantial, and it is hard to say sufficient.

SUMMARY OF THE INVENTION

(1) Objects of the Invention 5 One of the objects of the present invention is to provide a preferable liquid crystal display device for direct view type information display of OA apparatus by improvement of the above described problems. That is, to provide the liquid crystal display device having high 10 light transmittance at bright part, consequent high efficiency in utilization of incident light, and ability of high contrast display.

Other object of the present invention is to provide a preferable liquid crystal display device for reflection 15 type display.

Farther, other object of the present invention is to provide a display apparatus using the above described liquid crystal display devices.

(2) Methods Solving the Problems 20 The gist of the present invention to achieve the above

described objects is as follows:

(a) A liquid crystal display device characterized in comprising a set of substrates faced each other at least one of which is transparent, transparent electrodes pro- 25 vided respectively to each facing plane of said substrates, a light absorbing layer held between said transparent electrodes, and an electric power source for applying electric field to said light absorbing layer with said transparent electrodes, that said light absorbing 30 layer is composed of two layers, each of the layers has dichroism, average light absorbing axis of the each layers without electric field is approximately parallel to the substrate plane and crossing each other almost orthogonally, and said average light absorbing axis of the 35 each layers changes the axis direction by applying of electric field.

(b) A liquid crystal display device characterized in that one of the above described light absorbing layer is composed of polymer layer and liquid crystal, that is, 40 nematic liquid crystal having dichroism and positive dielectric anisotropy is dispersed in transparent polymer matrix, and the other light absorbing layer is a nematic liquid crystal layer surrounded with seal members and having dichroism and positive dielectric anisotropy. 45

(c) A liquid crystal display device characterized in that the above described light absorbing layer is composed of two polymer layers, each of the layers is composed of nematic liquid crystal particles having dichroism and positive dielectric anisotropy dispersed in trans- 50 parent polymer matrix, and said nematic liquid crystals

in each of said layers are aligned approximately parallel to the plane of said substrate in a specific orientation, and the orientation of said two layers alignment are so determined as to cross each other almost orthogonally. 55

(d) A liquid crystal display device characterized in comprising a set of substrates faced each other at least one of which is transparent, transparent electrodes provided respectively to each facing plane of said substrates, liquid crystal alignment control layers provided 60 on said transparent electrodes, two liquid crystal layers separated by a polymer layer and held between said set

of substrates, and seal portions surrounding vicinity of said liquid crystal layer, that said liquid crystal layer is nematic liquid crystal having dichroism and positive 65 dielectric anisotropy, said nematic liquid crystal is approximately parallel to the substrate plane and is aligned to a designated orientation, and the orientation of align

ment in said two layers of liquid crystal layers are crossing each other almost orthogonally.

In case of the composition of above described (b) and (c), it is preferable that a relative equation, n0=n^=n,;, is established among the refractive indexes of the nematic liquid crystal to ordinary light, n0, and to extraordinary light, ne, and the refractive index of the polymer matrix, np, and ideally n0=n^=nfis preferable.

The difference between the above described two refractive indexes, n0 and ne, of the nematic liquid crystal, namely, refractive index anisotropy An (=ne—n0), is necessary to be at most 0.1. The dielectric constant e//(dielectric constant of the liquid crystal in longitudinal direction) of the nematic liquid crystal is preferable to be approximately equal to the dielectric constant of the polymer matrix material, Cp, and large dielectric constant anisotropy, Ae (=ejy— ei), as possible is preferable for low voltage operation.

In case of the composition of the above described (d), preferable alignment of the two liquid crystal layers are obtained by forming the liquid crystal configuration control layers on both plane sides of the polymer layer, crossing each other of the direction of orientation on the plane sides almost orthogonally, and coinciding with the direction of orientation formed on the liquid crystal alignment control layer of the opposite side beyond the liquid crystal layer.

Also, same preferable configuration of the two liquid crystal layers are obtained by making the polymer layer have two layers structure composed of uniaxially stretched polymer material, making the stretching direction of polymer material of the two layers cross over almost orthogonally each other and coincide with the direction of oriented configuration formed on the liquid crystal alignment control layer of the opposite side beyond the liquid crystal layer.

The above described two liquid crystal layers preferably have almost same thickness, and thickness of the second polymer layer is preferably at most ten times of the thickness of the liquid crystal layer. If the thickness is larger than ten times, driving voltage increases remarkably. In order to control the thickness of the liquid crystal layer, spacer having a designated size is mixed into the liquid crystal layer and the seal portion, and the thickness of the liquid crystal layer is so controlled as to be uniform all through the panel.

As for the nematic liquid crystal, the one having small refractive index anisotropy is selected from liquid crystals prepared for the liquid crystal display of the TN type device and the STN type device at the present, and is used. For instance, liquid crystal composition including chemical compounds expressed by the following general formula as a main component: