JP2001005023A - Liquid crystal device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a liquid crystal device equipped with an optical writing type liquid crystal device having high optical writing sensitivity in which switching control for both directions of the liquid crystal is possible even in a panel structure having an insulating film having enough breakdown voltage. SOLUTION: A pair of substrates 11a, 11b are disposed facing each other at a specified distance in the optical writing liquid crystal device 1, and driving pulses which has an enough voltage to inverse the liquid crystal by a driving means 22 and enough long pulse width to allow the inner potential to an equilibrium state by inner discharge after inversion are applied on the transparent electrodes 12a, 12b formed on the transparent substrates 11a, 11b, respectively. The light irradiating means 21 to irradiate the optical writing liquid crystal device 1 with light for writing information is controlled by a controlling means 20 so that the light irradiating means 21 irradiates the device in the period after the inversion of the liquid crystal to the end of the driving pulse.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal device,
In particular, the present invention relates to a device having a light-writing type liquid crystal element which writes information by irradiating light.

[0002]

2. Description of the Related Art In recent years, TFTs and TN liquid crystal display elements of a passive matrix driving system have been known as widely used liquid crystal elements. However, a display device using the TN liquid crystal generally has a low response speed of the liquid crystal itself, and is not suitable for a moving image display device having a large screen and a high number of wirings, such as a liquid crystal television panel.

In order to solve such a fundamental problem of the TN liquid crystal, a liquid crystal using a chiral smectic liquid crystal having spontaneous polarization (hereinafter abbreviated as Ps) such as a ferroelectric liquid crystal or an antiferroelectric liquid crystal is used. Devices are known.
For example, a liquid crystal device using a ferroelectric liquid crystal has been proposed by Clark and Lagaval, and is disclosed in U.S. Pat.
No. 924.

Here, this liquid crystal element is different from a field effect type nematic liquid crystal element utilizing the dielectric anisotropy of liquid crystal molecules in that the polarity of Ps of the ferroelectric liquid crystal matches the polarity of the electric field. It switches between a first optically stable state and a second optically stable state.

Further, the ferroelectric liquid crystal has a property of maintaining the state when no electric field is applied, that is, has a bistable property.
Since a high-speed response on the order of c can be obtained, it is suitable for a large-screen display element that supports moving images.

However, in such a chiral smectic liquid crystal device, for example, “Structure and Physical Properties of Ferroelectric Liquid Crystal” (Corona Co., Atsuo Fukuda, Hideo Takezoe, eds. 199)
0), there was a problem that zigzag alignment defects were generated and the contrast was remarkably reduced. This defect is attributable to the fact that the layered structure of the chiral smectic liquid crystal carried between the substrates forms two types of chevron structures.

In recent years, there has been a movement to improve the chevron structure having such a defect and to realize a layered structure called a bookshelf or a structure similar thereto, thereby realizing a high-contrast and good liquid crystal element (for example, “ Next-generation liquid crystal displays and liquid crystal materials ", edited by CMC Corporation and Atsuo Fukuda, 1992).

A liquid crystal material having a perfluoroether side chain (US Pat. No. 52620) is a liquid crystal material exhibiting such a bookshelf or a structure close thereto.
82, International Application Patent WO93 / 22396, 1993 Fourth International Conference on Ferroelectric Liquid Crystal P-46, Mark D. Ra
dccliffe et al.). In addition, this liquid crystal can exhibit a bookshelf or a structure having a small layer tilt angle close thereto without using an external field such as an electric field, and is suitable for a high-speed, high-definition, large-area liquid crystal element and a display device. I have.

On the other hand, as an element structure for favorably aligning the liquid crystal material, an alignment control layer is formed on a pair of substrates sandwiching a liquid crystal, as described in, for example, JP-A-61-20930. A configuration in which one orientation control layer is subjected to uniaxial orientation treatment and the other is provided with an asymmetric orientation control layer not subjected to uniaxial orientation treatment is provided. Then, by performing a uniaxial alignment process on one of the alignment control layers and not performing a uniaxial alignment process on the other,
The orientation of the liquid crystal can be controlled in a high order from the side subjected to the uniaxial orientation treatment, and a favorable liquid crystal alignment state can be easily obtained.

FIG. 6 is a view showing such an element configuration. In FIG. 6, reference numerals 61 and 62 denote glass substrates, 63 and 6
4 is a transparent electrode, 65 and 66 are first alignment films, 67 is a chiral smectic liquid crystal, and 68 is a uniaxially oriented second liquid crystal.
Of the alignment film.

However, in the liquid crystal element having such a configuration, the presence of Ps in the chiral smectic liquid crystal induces uneven distribution of electric charges in the liquid crystal element, causing a switching failure, which is a so-called anti-electric field problem.

Next, an anti-electric field when a ferroelectric liquid crystal is used will be described with reference to FIGS. 7 and 8. FIG. In FIG. 7, reference numeral 73 denotes a liquid crystal molecule, reference numerals 71 and 72 denote two memory states of ferroelectric liquid crystal, and .times. Indicates the direction of Ps. P1 and P2 are the liquid crystal molecules 73
7 shows an element applied voltage waveform for switching the state from the state 71 to the state 72 or from the state 72 to the state 71, respectively.

On the other hand, FIG. 8A shows a voltage waveform applied directly to the chiral smectic liquid crystal 67, and FIG.
4 is a liquid crystal voltage drop due to Ps inversion, and 75 is an anti-electric field generated immediately after the short circuit. (B) shows the optical response of the liquid crystal corresponding to the voltage waveform shown in (a).

Here, the initial voltage value of the anti-electric field (hereinafter referred to as Vr
ev) is equal to the value of 74 and decays with a time constant τ. This can be expressed by the following equation.

Inverse electric field = Vrev · e ^{−t / T} (Equation 1) In the above equation 1, Vrev = 2Ps · S / (Clc + Cins) T = (Clc + Cins) · (Rlc · Rins / (R
lc + Rins)) S: electrode area, Clc, Rlc: liquid crystal capacitance and resistance, Cins, Rins: laminated film capacitance and resistance, and the laminated film capacitance Cins and resistance Rins are shown in FIG.
Of the first and second alignment films 65 and 66 and the second alignment film 68.

As shown in FIG. 8A, when the anti-electric field -Vrev is smaller than the threshold value Vrev-th of the re-inversion (from state 72 to 71), the liquid crystal inversion (from state 71 to 72). If (v) is completed and the anti-electric field -Vrev is larger than the re-inversion threshold value Vrev-th, re-inversion occurs and switching failure occurs.

In order to suppress the anti-electric field, examples of lowering the impedance of the orientation control layer are disclosed in JP-A-63-121020 and JP-A-64-49023.
This is disclosed in Japanese Patent Publication No. In order to apply this to the configuration shown in FIG. 6, for example, Ps is 10 nC / c
When polyimide or polyamide is used for the liquid crystal 67 of m ² or more and the second alignment film 68, the volume resistance of the first alignment films 65 and 66 is set to 1 × 10 ⁴ to 1 × 10 ¹⁰ Ωcm, and the second alignment film is formed. It is desirable that the thickness of the film 68 be 200 ° or less.

As described above, in the conventional chiral smectic liquid crystal element, the first alignment films 65 and 66 and the second alignment film 68 which also have the function of preventing short-circuit as described above are fixed to a switchable impedance. In the above (anti-electric field <Vrev-th), switching control is performed by applied voltage modulation or pulse width modulation.

In such a conventional liquid crystal element, for example, in the configuration shown in FIG. 6, in order to suppress the counter electric field, the first alignment films 65 and 66 have low impedance, for example, It is desirable to form a film in which conductive fine particles are dispersed in an inorganic binder. When the first alignment films 65 and 66 have a low impedance, the first alignment films 65 and 66 have a low withstand voltage. , Short-circuiting easily occurs.

Then, when such a short circuit occurs,
In the case of a TFT driving element, a bit defect is caused, and in the case of a passive matrix driving element, a line defect is caused. Note that a liquid crystal element using a film having a sufficient withstand voltage characteristic, for example, SiO ₂ , Ta ₂ O _5, or the like as an insulating film and using a liquid crystal of low Ps (several nC / cm ² ) has a switching speed and a memory. This causes a problem of deterioration in performance.

[0021]

As an example of a liquid crystal device having such a structure, a liquid crystal is sandwiched between a pair of transparent substrates opposed to each other at a predetermined distance, and an incident light amount is incident on one of the transparent substrates. There is a photo-writing type liquid crystal element in which a photoconductive film whose volume resistance changes due to light is applied and data is written by irradiating light.

Here, in such a photo-writing type liquid crystal element, a thick photoconductive film is formed so that the capacitance value of the photoconductive film is substantially the same as the capacitance value of the liquid crystal, and the capacitance value is increased. , The voltage of the anti-electric field becomes larger.

Thus, when optical writing is performed (in a state where light is applied), the photoconductive film is lowered in resistance and a voltage is directly applied to the liquid crystal, but in a dark state (in a state where light is not applied, (Or in areas that have not been exposed to light) the large anti-electric field makes it impossible to switch the liquid crystal. That is, the optical writing type liquid crystal element has a configuration in which an anti-electric field is actively utilized.

FIG. 9 is a diagram showing the configuration of such a conventional optically-writing type liquid crystal element. In FIG.
1b is a glass substrate, 82a and 82b are transparent electrodes, 83 is a charge generation layer, 84 is a charge transport layer, and the charge generation layer 83
And the charge transport layer 84 constitute a photoconductive film 83A.

Reference numeral 85 denotes a passivation film of an inorganic binder material, 86 denotes a second alignment film that has been subjected to a uniaxial alignment treatment,
87 is a chiral smectic liquid crystal. Note that the passivation film 85 prevents the charge transport material from seeping out of the charge transport layer 84 into the chiral smectic liquid crystal, and has a role as an alignment control film that is not subjected to a uniaxial alignment treatment.

Here, since the holes mainly transported in the charge transport layer 84 are positive holes, when the liquid crystal element is irradiated with sufficient light from the outside, the transparent electrode 82a side becomes the unipolar light of the positive electrode side. Functions as a conductive film. That is, holes (not shown) are supplied from the charge generation layer 83 to the charge transport layer 84 by light irradiation, and the holes flow through the charge transport layer 84 by an external electric field, so that a current flows.

However, when an electric field is applied such that the transparent electrode 82a is on the negative electrode side, no current flows because no holes are supplied from the passivation film 85 by light irradiation. Therefore, in the case where the liquid crystal 87 is switched by applying an electric field with the transparent electrode 82a as the negative electrode side, even when irradiated with light, the photoconductive film 83A does not exhibit conductivity, so that the photoconductive film 83A affects the liquid crystal driving as a capacitance component. , The effect of the anti-electric field increases. That is, when the liquid crystal is switched by applying an electric field of the opposite polarity, switching failure of the liquid crystal is caused.

Therefore, the present invention has been made in view of such a situation, and enables switching control of liquid crystal in both directions even in a panel configuration in which an insulating film having a sufficient withstand voltage is provided, and optical writing. It is an object of the present invention to provide a liquid crystal device having an optical writing type liquid crystal element having high sensitivity.

[0029]

SUMMARY OF THE INVENTION The present invention provides a pair of transparent substrates, which are opposed to each other at a predetermined distance, a transparent electrode formed on each of the pair of transparent substrates, and a transparent electrode formed on one of the transparent substrates. And a photoconductive film whose resistance value changes according to the amount of incident light, an alignment film formed on at least the other transparent substrate, and a liquid crystal sandwiched between the pair of transparent substrates. A liquid crystal element for writing information in response thereto, a voltage sufficient for inverting the liquid crystal on the pair of transparent substrates, and a voltage sufficient for the internal potential to transition to an equilibrium state by internal discharge after the inversion. A driving unit for applying a driving pulse having a long pulse width, a light irradiating unit for irradiating the light-writing type liquid crystal element with light for writing information, and a light irradiating device between the liquid crystal inversion and the end of the driving pulse. And control means for controlling the light emitting means to perform, is characterized in that it comprises.

Further, in the present invention, the control means preferably controls the light irradiation means so as to perform light irradiation after at least a half of a pulse width of the drive pulse has been applied after the drive pulse is applied. It is a feature.

Further, the present invention is characterized in that the control means controls the light irradiation means so that the light irradiation time is shorter than the driving pulse width by one digit or more.

The present invention is also configured such that the photoconductive film is functionally separated into a charge generation layer and a charge transport layer, and the resistance value varies depending on the direction of an electric field applied between the pair of transparent electrodes. It is characterized by having been done.

Further, the present invention is characterized in that a low impedance passivation layer is laminated on the charge transfer layer of the photoconductive film.

Further, the present invention is characterized in that the liquid crystal is a ferroelectric liquid crystal or an antiferroelectric liquid crystal.

As in the present invention, a voltage sufficient for inverting the liquid crystal by the driving means is applied to the transparent electrodes formed on a pair of transparent substrates opposed to each other at a predetermined distance from the optical writing type liquid crystal element. After the inversion, a drive pulse having a pulse width long enough to cause the internal potential to transition to an equilibrium state by the internal discharge is applied. Further, the control means controls light irradiation means for irradiating the light-writing type liquid crystal element with light for writing information, so that the light irradiation means performs light irradiation after the liquid crystal inversion until the end of the driving pulse. I do.

[0036]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a plan view schematically showing a liquid crystal device according to an embodiment of the present invention, and FIG. 2 is a sectional view taken along line BB ′ of FIG.
It is a line sectional view.

1 and 2, reference numeral 1 denotes a light-writing type liquid crystal element (hereinafter, referred to as a liquid crystal element), 2 denotes a liquid crystal device, and 11 denotes a liquid crystal device.
Reference numerals a and 11b denote a pair of substrates opposed to each other at a predetermined distance, and the pair of substrates 11a and 11b
8, the liquid crystal 16 is sandwiched therebetween, and an effective optical modulation area (corresponding to a display area in a display element) S1 between the substrates 11a and 11b and the sealing material 18 and a peripheral area (corresponding to a non-display area in the display element) ) S2 is formed.

The transparent electrodes 12a and 12b are formed on the pair of substrates 11a and 11b. Here, it is desirable that the transparent electrodes 12a and 12b are formed of a transparent conductor.
Tin oxide, indium oxide, indium tin oxide (ITO)
Etc. are preferably used. Further, a configuration in which a metal layer having a lower resistance is provided to the transparent conductor as necessary may be employed. Further, its thickness is desirably set to 40 to 200 nm.

On the other hand, reference numeral 13 denotes an alignment film provided on the upper substrate 11a, and this alignment film 13 is provided in a portion corresponding to the peripheral region S2 and the effective optical modulation region S1 of the lower substrate 11b. The orientation film 13 is oriented in a direction corresponding to a specific condition in order to orient the liquid crystal 16 horizontally or at an appropriate pretilt angle with respect to the substrate 11a in a portion corresponding to the effective optical modulation region S1 (for example, FIG. 1). (Direction shown in FIG. 3) is subjected to a uniaxial orientation treatment such as rubbing.

The material of the alignment film 13 includes polyimide, polypyrrole, polyvinyl alcohol, polyamide imide, polyester imide, polyparaxylene, polyester, polycarbonate, polyvinyl acetal, polyvinyl chloride, polyamide, polystyrene,
An organic film such as polyaniline, cellulose resin, acrylic resin, and melamine resin, or an inorganic film such as an oblique vapor deposition film of SiO can be appropriately selected. In the oblique evaporation method, the uniaxial orientation processing direction can be set by controlling the evaporation direction. It is desirable that the thickness be set to 5 to 100 nm.

In FIG. 2, reference numeral 14 denotes the lower substrate 1
1b is a photoconductive film that is a photodiode film formed of an amorphous film and provided in a portion corresponding to the effective optical modulation region S1. The photoconductive film 14 is a charge generation layer (Carrier Generation Layer).
r, hereinafter CGL) 14a and a charge transport layer (Car
rier Transfer Layer, CTL
14b).

The CGL 14a is obtained by dispersing particles of a photoconductive substance (charge generating substance) in a resin.
Examples of the photoconductive material include compounds having an electron-donating part and an electron-accepting part in the molecule, for example, azo pigments such as sudanlet and diane blue, and phthalocyanine pigments such as copper phthalocyanine and titaninolephthalocyanine. Examples of the resin for dispersing these particles include a binder resin such as polyvinyl butyral, polystyrene and polyvinyl acetate. The thickness of the CGL 14a is 5
It is desirable to set it to 0 to 200 nm.

On the other hand, the CTL 14b is obtained by dissolving a charge transporting substance having a large carrier mobility in a resin having a film forming property. The charge transporting substance includes a main chain or a side chain such as biphenylene or anthracene pyrene. And a nitrogen-containing heterocyclic compound such as indole and carbazole, and a hydrazone compound. Examples of the resin having a film forming property include polyester resin, polycarbonate, and polystyrene. In addition,
It is desirable that the thickness of the CTL 14b be set to 200 to 20,000 nm.

Further, the photoconductive film 14 has a CTL 14
A protective film 15a, which is a passivation film that performs the function of passivation b, and a second alignment film 15b that is not subjected to a uniaxial alignment process are formed.

Here, this protective film 15a is
This is to prevent the charge transport material as an impurity from seeping out into the liquid crystal to contaminate the liquid crystal 16 and degrade its characteristics. Materials are undesirable. Therefore, in the present embodiment, a general SiO _x polymer inorganic binder is used as the inorganic coating agent.
The thickness of the protective film 15a is desirably set to 7 to 15 nm.

On the other hand, the second alignment film 15b does not have a regulating force on the alignment of the liquid crystal 16 by not performing the uniaxial alignment process, but has been described above by performing the uniaxial alignment process on one of the alignment films 13. As described above, the orientation of the liquid crystal 16 can be controlled in a high order from the side subjected to the uniaxial orientation treatment, and a favorable orientation state can be easily obtained. The thickness of the protective film 15b is desirably set to 10 to 20 nm.

Further, the second alignment film 15b has a thickness of about 1
The volume resistance value is set to × 10 ⁶ Ωcm. Here, such an orientation film 15b is suitably obtained by specifically forming a film in which conductive fine particles are dispersed in an organic or inorganic binder. In addition, polyimide and polycarbonate are preferably used as the organic binder, and a polymer of SiO _x is preferably used as the inorganic binder. Further, as the conductive fine particles, SnO
_x and TiO _x are preferably used.

In this embodiment, the protective film 15a and the second alignment film 15b are formed separately from each other.
A single layer may be formed by using a film in which conductive fine particles are dispersed in an inorganic binder.

As the liquid crystal 16 sandwiched between the substrates, for example, a chiral smectic liquid crystal, which is a liquid crystal in which the action of spontaneous polarization is applied to driving, such as a ferroelectric liquid crystal or an antiferroelectric liquid crystal, is used.

Here, as such a ferroelectric liquid crystal,
For example, from high temperature to low temperature, isotropic → SmA
A liquid crystal having a phase transition series of (Smectic A phase) → SmC * → Crystal phase is used. As the liquid crystal suitably used in the present invention, the above-mentioned liquid crystal compound having a perfluoroether side chain is suitable. Also,
In the present embodiment, as described above, Ps is 10 nC / c.
Although a liquid crystal of m ² is used, a material having a large Ps may be used according to the application of the element, for example, when the switching speed is increased.

In FIG. 2, reference numeral 17 denotes silica beads having an average particle diameter of about 2.8 μm dispersed in the liquid crystal, and the silica beads 17 keep the distance between the substrates constant. Further, a plurality of bead-shaped adhesives (not shown) may be dispersed in the liquid crystal in order to adhere the substrates 11a and 11b.

On the other hand, in FIG. 1, 22 is a substrate 11a,
Driving means for applying a predetermined driving pulse between the transparent electrodes 12a and 12b formed on 11b, 21 is light irradiating means for irradiating the effective optical modulation area S1 with light to add two-dimensional optical data, and 20 is driving means 22 Control means for controlling the timing between the light irradiation means 21 and the light irradiation means 21. The light irradiation means 21
Discloses a method in which light from a light source such as a strobe flash is light-modulated through a developed negative film or the like, and is enlarged and projected onto an effective optical modulation area using an enlargement optical system, thereby performing light irradiation.

In the liquid crystal device 2, when writing a display image, the writing light emitting device 26 is applied with a predetermined voltage between the transparent electrodes 12a and 12b, for example, a positive voltage applied to the transparent electrode 12b. Irradiates the liquid crystal element 1 with light.

Here, at the portion where light is written, CGL1
An electric charge is generated at 4a, and the electric charge moves in the CTL 14b by the applied voltage and moves to the vicinity of the protective film 15a. Then, a high electric field is applied to the liquid crystal 16 only at a portion irradiated with light due to such movement of the charge, so that the liquid crystal 16 at the portion is inverted. It should be noted that the amount of the liquid crystal 16 to be inverted at the light-irradiated portion changes depending on the intensity of the irradiated light. Therefore, the transmittance of the liquid crystal 16 at the time of display can be easily modulated by the intensity modulation of the writing light. Becomes possible.

Next, the switching performance of the optical writing type liquid crystal element according to the present invention will be described.

FIG. 3 shows a dark state (a state in which light irradiation is not performed).
FIG. 2 is a diagram showing an equivalent circuit of the liquid crystal element 1 in FIG. 1, where C _OPC is the electric capacity of the photoconductive film (OPC) 14 and R
_1OPC and R _2OPC are resistance values in the thickness direction of the photoconductive film 14 and C _LC
The capacitance of the liquid crystal layer, C _Ps is the electric capacity corresponding to the spontaneous polarization of the liquid crystal 16, the R _LC is the resistance value in the thickness direction of the liquid crystal layer. Note that, in the present embodiment, the protective film 15a and the second alignment film 15b are set to be thin, and have a volume resistance value that is negligibly small and a capacitance value that is negligible.

By the way, in this embodiment, the photoconductive film 14 depends on D ₁ and D ₂ shown in FIG.
Have different resistance values. Here, in order to make the resistance of the photoconductive film 14 different depending on the direction of the electric field, in the present embodiment, as described above, the photoconductive film 14 is used.
CGL14a having low resistance to electron movement
And CTL1 with low resistance to hole movement
A function-separated two-layer structure having a layer 4b is provided.

When the photoconductive film 14 has a two-layer structure of the CGL 14a and the CTL 14b having such characteristics, carrier recombination is likely to occur at the CGL / CTL interface, but carrier generation (separation) occurs. The resistance of the photoconductive film 14 varies depending on the direction of the electric field.

That is, when a negative voltage is applied to the transparent electrode 12b (hereinafter referred to as Reset), the protective film 15
Holes injected from a move in the CTL 14b, electrons move in the CGL 14a, and carrier recombination occurs at the CGL / CTL interface, resulting in a low resistance value. On the other hand, when a positive voltage is applied to the transparent electrode 12b (hereinafter referred to as writing), carriers are not generated at the CGL / CTL interface unless light is irradiated.
Carrier movement in L14a and CTL14b is small and the resistance value is high.

Further, in order to make the resistance value of the photoconductive film 14 different depending on the direction of the electric field, in this embodiment, the volume resistance value and the film thickness of the protective film 15a are changed.

Here, only when holes are injected from the protective film 15a into the CTL 14b at the time of Reset, the CT
The hole moves in L14b, and CGL14a and CTL1
A current flows due to recombination with electrons at the interface with the electrode 4b. At this time, if injection of holes from the protective film 15a is sufficient, injection of electrons from the transparent electrode 12a is sufficient. 14 has low resistance characteristics.

Conventionally, in order to invert the liquid crystal 16, a driving pulse having a pulse width sufficient for inverting the liquid crystal 16 or a pulse width sufficient for moving the hole in the CTL 14b by light irradiation is applied. It was thought that it was enough.

For example, the switching speed (τ) of the chiral smectic liquid crystal 16 with Ps = about 10 nC / cm ² is τ <100 μsec and CTL1
The carrier mobility (μp) of 4b is 1 × 10 ⁻⁵ to 10
^-7 (cm ² / V · sec), and the movement time t determined by t <5 msec, it has been considered that a drive pulse width of 5 msec is sufficient.

Further, it is considered that the driving voltage at the time of optical writing may be a voltage sufficient for inverting the liquid crystal only after the photoconductive film 14 has a low resistance due to light irradiation and the driving voltage is completely applied to the liquid crystal. The voltage was 5-8V.

However, in such a driving method, the charge is not sufficiently relaxed after inversion of the liquid crystal, and the sensitivity of optical writing cannot be increased. Therefore, in the present invention,
In order to increase the sensitivity of optical writing, after the drive voltage is applied, the pulse width is long enough to end the drive pulse after a sufficient time has elapsed after the liquid crystal has been inverted,
In addition, driving is performed at such a high voltage that a sufficient electric field capable of inverting the liquid crystal can be applied to the liquid crystal without the photoconductive film having a low resistance.

By adopting such a driving method, the generated and injected charges are alleviated according to the CR time constant shown in FIG. 3 during a sufficient time after the inversion of the liquid crystal. It is possible to increase the sensitivity.

FIG. 4 is a diagram showing the switching characteristics of the liquid crystal when the panel is driven with a pulse width (40 to 200 msec) sufficiently longer than 5 msec in the dark.
(B) shows a change in the liquid crystal transmittance, a driving waveform, and an electric field waveform applied to the liquid crystal at the time of reset and at the time of writing, respectively. In the present embodiment, the orientation of the polarizing plate is set so that the transmitted light of the liquid crystal at the time of Reset is maximized.

At the time of Reset, the drive voltage shown in FIG.
At the moment when an electric field is applied at 0 to 40 V, the capacitance is divided by the electric capacitance C _OPC of the photoconductive film 14 and the electric capacitance C _LC of the liquid crystal layer, and about half the electric field is applied to the liquid crystal layer (in FIG. Is an electric field applied to the liquid crystal). Here, since the threshold value of the liquid crystal 16 is several volts, all the domains are inverted by this electric field, and a voltage drop due to Ps inversion occurs.

On the other hand, since the switching speed of the liquid crystal is at least two orders of magnitude faster (several tens of μsec) for a pulse width of 40 to 200 msec, the switching of the liquid crystal is completed at the first stage in the driving waveform. When the inversion of the liquid crystal is completed, the internal potential gradually changes to an equilibrium state due to the internal discharge caused by the movement (relaxation) of the internal charge in the photoconductive film and the liquid crystal layer, and the electric field applied to the liquid crystal changes to the bias point A. I do.

Here, the volume resistivity ρ of the chiral smectic liquid crystal 16 (the liquid crystal compound having a perfluoroether side chain described above) is generally 1 × 10 ¹² to 10 ¹³ (Ωcm).
In contrast, the apparent volume resistance of the _{CTL 14b} is R 1 _OPC ≒ 1 × 10 ¹¹ (Ωcm) and R 2 _OPC ≒ 10
It is set to be ¹³ (Ωcm).

The R 2 _OPC is set by selecting the material of the CGL 14 a and securing the injection amount of the thermally excited carrier at the interface.
Since the injection amount changes due to the difference between the conduction level of a and the CTL 14b = energy barrier, it can be controlled by a combination of both. The setting of R 1 _OPC is performed by controlling the volume resistance value and the film thickness of the protective film 15 a in order to improve the performance of injecting holes from the protective film 15 a into the CTL 14 b.

Accordingly, the time required for relaxation is as follows: the dielectric constants of the liquid crystal 16 and the photoconductive layer 14 are 3 to 4, so that R _1OPC ,
Time constant of the R _LC is about several 10 msec. Therefore, if the drive pulse width is about 100 msec, a sufficiently relaxing time is secured.

On the other hand, the bias point A is a value _{divided according to} the resistance value R ₁ OPC in the thickness direction of the _{photoconductive film} and the resistance value R _LC in the thickness direction of the liquid crystal layer, and R ₁ OPC≪R _LC. If the resistance value R 1 _OPC in the thickness direction of the photoconductive film is set, most of the electric field is applied to the liquid crystal layer at the bias point A, so that the influence of the counter electric field is small and all the liquid crystals complete the inversion. . However, if the drive pulse ends before the bias point A is reached, the electric field applied to the liquid crystal layer is at a stage where the electric field applied to the liquid crystal layer is small, as shown in FIG. This causes switching failure.

On the other hand, at the time of writing, in order to bring the liquid crystal to a black state (transmittance: 0%), the same applies to the inversion operation of the liquid crystal as shown in FIG. The value of B is the resistance R in the thickness direction of the photoconductive film 14.
There value is divided according to the thickness direction of the resistance value R _LC of _2OPC and the liquid crystal layer, by setting the R _2OPC ≒ R _LC and so as photoconductive film 14 resistance R _2OPC in the thickness direction of the bias Point B
As a result, about half of the drive voltage is applied to the liquid crystal layer.

As a result, at the moment when the electric field is cut off, a large reverse electric field is applied to the liquid crystal due to the anti-electric field, and the liquid crystal is again inverted by the electric field and returns to the initial white state. That is, in the dark state (when there is no light irradiation), in the case of writing, if a driving voltage is applied with a sufficiently long pulse width, the liquid crystal is not switched due to re-inversion by an anti-electric field, and only when light is irradiated. Is realized, the operation of “optical writing” is realized.

Next, the operation during light irradiation will be described.

First, carriers are generated at the interface between the CGL 14a and the CTL 14b by light irradiation by the light irradiating means 21, and holes are generated in the state where an electric field is applied.
Into the CTL 14b according to the electric field.
Move in the direction. And, by the movement of this hole, C
The TL 14b functions as a conductive layer, and the voltage applied to the liquid crystal 16 increases according to the amount of moving carriers (holes).

Since the light irradiation time is about 1 msec, which is shorter than the driving pulse width, if the light irradiation is performed at the first half timing of the driving pulse, the movement of the carrier is completed within the driving pulse period. Then, the electric field applied to the liquid crystal due to the start of the relaxation of the carriers approaches the bias point B with the passage of time, and the effect of light irradiation is lost.

For this reason, in the present embodiment, the control means 20 adjusts the light irradiation timing to 50 to 50% in the latter half of the drive pulse, for example, when the drive pulse width is 100 msec after the drive pulse is applied and the liquid crystal is inverted. The light irradiation means is controlled to perform light irradiation with a delay of 80 msec.

FIG. 5 is a view showing the result of optical writing performed by setting a polarizing plate so that the transmittance becomes minimum (about 0%) before optical writing. In the figure, the horizontal axis represents the irradiation light intensity, and the vertical axis represents the transmittance after writing. The transmittance when all the liquid crystals are inverted is set to 100%.

Further, in the same figure, the broken line shows the data when the driving pulse width is set to 5 msec, the light irradiation is performed simultaneously with the driving, and the solid line shows the driving pulse width when the driving pulse width is 100 msec.
c and data when light irradiation is performed with a delay of 80 msec from the drive, respectively, and shows that the steeper the data changes, the higher the light sensitivity to a change in light.

As is apparent from the figure, by driving with a sufficiently long pulse width and setting the timing of light irradiation to the latter half of the driving pulse, the sensitivity at the time of optical writing can be increased, whereby the light It can be seen that the effect of irradiation is maximized and a liquid crystal element with high photosensitivity can be obtained.

[0084]

As described above, as in the present invention, a voltage sufficient for inverting the liquid crystal in the optical writing type liquid crystal element and a voltage sufficient for the internal potential to transition to an equilibrium state by internal discharge after the inversion are obtained. By applying a drive pulse having a long pulse width to the light source and irradiating light from after the inversion of the liquid crystal to the end of the drive pulse,
At the time t, not only the switching failure due to the influence of the anti-electric field can be suppressed, but also the optical writing sensitivity can be improved at the time of optical writing.

[Brief description of the drawings]

FIG. 1 is a plan view schematically showing a liquid crystal device according to an embodiment of the present invention.

FIG. 2 is a sectional view taken along line B-B 'of FIG.

FIG. 3 is a diagram showing an electrical equivalent circuit of a light-writing type liquid crystal element of the liquid crystal device.

FIG. 4 is a diagram showing an electric field applied to the liquid crystal and a switching characteristic of the liquid crystal in a dark state of the optical writing type liquid crystal element.

FIG. 5 is a diagram showing a relationship between a light amount and a light transmittance of the optical writing type liquid crystal element according to driving conditions.

FIG. 6 is a cross-sectional view illustrating a schematic configuration of a conventional liquid crystal element.

FIG. 7 is a diagram illustrating switching of the above-described conventional liquid crystal element by an anti-electric field.

FIG. 8 is a diagram illustrating a switching failure of the above-described conventional liquid crystal element due to an anti-electric field.

FIG. 9 is a cross-sectional view showing a configuration of a conventional optical writing type liquid crystal element.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Optical writing type liquid crystal element 2 Liquid crystal device 11a, 11b Glass substrate 12a, 12b Transparent electrode 13 Alignment film which performed uniaxial alignment processing 14 Photoconductive film 14a Charge generation layer 14b Charge transport layer 15a Protective film 15b 2nd alignment film 16 (chiral smectic) liquid crystal 20 control means 21 light irradiation means 22 drive means

Claims (6)

[Claims] 1. A pair of transparent substrates opposed to each other at a predetermined distance, a transparent electrode formed on each of the pair of transparent substrates, and a resistance value formed on one of the transparent substrates and based on an incident light amount. A photoconductive film that changes the light, an alignment film formed on at least the other transparent substrate, and a liquid crystal interposed between the pair of transparent substrates, and a light for writing information in accordance with the irradiated light. A drive pulse having a voltage sufficient for inverting the liquid crystal on the pair of transparent substrates, and a pulse width long enough for the internal potential to transition to a balanced state by internal discharge after the inversion. A light irradiating means for irradiating the light-writing type liquid crystal element with light for writing information; and a light irradiating means for irradiating light from the liquid crystal inversion to the end of the driving pulse. A liquid crystal device, comprising: control means for controlling a stage. 2. The control means controls the light irradiating means so as to perform light irradiation after at least half of a pulse width of the driving pulse has elapsed after the application of the driving pulse. The liquid crystal device according to claim 1. 3. The liquid crystal device according to claim 1, wherein the control unit controls the light irradiation unit such that the light irradiation time is shorter than the driving pulse width by one digit or more. 4. The photoconductive film, wherein the photoconductive film is functionally separated into a charge generation layer and a charge transport layer, and the resistance value varies depending on a direction of an electric field applied between the pair of transparent electrodes. The liquid crystal device according to claim 1, wherein 5. The liquid crystal device according to claim 1, wherein a low-impedance passivation layer is laminated on the charge transfer layer of the photoconductive film. 6. The liquid crystal device according to claim 1, wherein the liquid crystal is a ferroelectric liquid crystal or an antiferroelectric liquid crystal.