JPH0416097B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a liquid crystal device used for a liquid crystal display element, a liquid crystal-light shutter, etc.
By improving the initial alignment state of liquid crystal molecules,
This invention relates to a liquid crystal device with improved display and driving characteristics.

[Summary of the Disclosure] This specification and drawings describe a ferroelectric liquid crystal element used in a liquid crystal display element, a liquid crystal-light shutter, etc., and a method for driving the same, in which one side of the bistability of the ferroelectric liquid crystal is controlled by an applied voltage. It appears when an electric field that exceeds a predetermined threshold value and voltage application time is applied, and when there is no electric field, it is relaxed to the other stable state. The present invention discloses a technology that enables high-speed display suitable for moving images on television etc. by applying an electric field exceeding the threshold value and driving unselected pixels by applying an electric field below the threshold value. be.

[Prior Art] Conventionally, liquid crystal display elements display images or information by configuring a scanning electrode group and a signal electrode group in a matrix, and filling a liquid crystal compound between the electrodes to form a large number of pixels. well known. The driving method for this display element is time-division driving, in which an address signal is selectively and periodically applied to a group of scanning electrodes, and a predetermined information signal is selectively applied in parallel to a group of signal electrodes in synchronization with the address signal. It has been adopted.

On the other hand, looking at the field of printers, laser beams, which provide electrical image signals in the form of light to an electrophotographic photoreceptor, are effective in terms of both pixel density and speed as a means of obtaining hard copies using electrical signals as input. Printers are currently the best. A liquid crystal shutter array has been proposed as an element that converts the electrical signal into an optical signal.

Most of the materials used in these practical applications were, for example, “Applied Physics Letters.”
(“Applied Physics Letters”) 1971, No. 18(4)
“Voltage dependent optical activity of a twisted nematic liquid crystal” co-authored by M. Sshadt and W. Helfich, published on pages 127-128. (“Voltage
Dependent Optical Activity of a Twisted
Nematic Liquid Crystal”)
(twisted nematic) type liquid crystal.

In recent years, the use of bistable liquid crystal elements as an improved version of conventional liquid crystal elements has been proposed by both Clark and Lagerwall in Japanese Patent Application Laid-open No. 107216/1983 and US Patent No.
It has been proposed in the specification of No. 4367924, etc. As a bistable liquid crystal, a ferroelectric liquid crystal having a chiral smectic C phase (SmC ^* ) or H phase (SmH ^* ) is generally used. 1 optical state and the 2nd optical state
The property of taking one of the optically stable states of and maintaining that state when no electric field is applied;
That is, it has stability and quick response to changes in electric field, and is expected to be widely used in fields such as high-speed and memory-type display devices.

[Problems to be Solved by the Invention] However, when trying to display a moving image in which the screen changes sequentially, such as a television image, using such a ferroelectric liquid crystal, the liquid crystal exhibits the above-mentioned bistability. Therefore, each time the screen is rewritten, an electric field of opposite polarity is required, which makes the driving method complicated and has the disadvantage that it takes a long time to write one screen.

An object of the present invention is to solve the above-mentioned problems in ferroelectric liquid crystal elements having bistability and to provide a liquid crystal device using a novel ferroelectric liquid crystal element.

[Means for Solving the Problems] The present invention provides: (a) a matrix electrode that uses a plurality of pixels formed at the intersections of a scanning electrode group and a signal electrode group in a display screen; and one stable state, which appears when an electric field exceeding the threshold is applied, and the above-mentioned one when no electric field is applied.
a liquid crystal cell having a ferroelectric liquid crystal that produces two states, one metastable state that transitions relaxedly to two stable states; The scanning is performed so that, after the liquid crystal molecules have transitioned to a metastable state, the relaxation time for returning from the metastable state to the stable state is longer than the time required to form one screen by scanning the scanning electrode group. The liquid crystal device is characterized by a means for applying a voltage to an electrode group and a signal electrode group.

[Function] For a chiral smectic liquid crystal that originally has a spiral structure, the spiral can be unwound by reducing the distance between the substrates of the liquid crystal cell, for example, 3 μm or less.
Obtaining a bistable state is known as a surface-stable memory cell. (Unexamined Japanese Patent Publication No. 107216/1983). FIG. 1 is a schematic diagram showing an example of such a surface-stabilized ferroelectric liquid crystal cell (SSFIC cell). In the figure, a thick line 23 represents a liquid crystal molecule, and this liquid crystal molecule 23 has a dipole moment 24 (P⊥) in a direction perpendicular to the molecule. When a voltage equal to or higher than a certain threshold is applied between the electrodes on the pair of substrates 11 and 11', the alignment direction of all the dipole moments 24 can be changed so that they are oriented in the electric field direction E or E'.
The two orientation directions 23 and 23' of the liquid crystal molecules are bistable and energetically equivalent.

The present inventors investigated in detail the relationship between the orientation control method and bistability in SSFLC cells, and found that by using an appropriate orientation control method, two states as shown in Figure 1 can exist. Regardless, one state is more stable than the other, and by applying a voltage above a certain threshold, the crystal molecules can be oriented to the other state opposite to one stable state. We also discovered that there is a metastable state that quickly returns to the original stable state after the voltage is removed. The details of the appearance of such a metastable state are not clear, but due to some kind of asymmetrical orientation control action in the upper and lower substrates, the energies of the two bistable states are no longer equivalent, resulting in a stable state and a metastable state. It is thought that a situation like this will occur.

[Example] Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

FIG. 2 is a waveform chart showing an example of the optical response of a liquid crystal element embodying the present invention. In Figure 2,
The horizontal axis shows time, and the vertical axis shows the voltage value of the applied voltage and the amount of transmitted light.

The crystal used in this example is CSS3MIX, which has the following molecular formula.

Mix 10% of the following substance with the above liquid crystal, After applying a polymer film such as polyimide to a glass substrate coated with ITO or the like, a cell was formed by applying an alignment treatment to make the rubbing direction perpendicular to the upper and lower substrates. Here, the cell thickness is desirably 3 μm or less, preferably up to 1 μm. The amount of transmitted light is
When a liquid crystal is arranged between two polarizing plates in a crossed nicol relationship so that it is at its darkest level in the stable state, and a predetermined pulse voltage is applied to switch to the metastable state. The amount of transmitted light was measured using a photomultiplier tube.

Figure 2a shows the case where a pulse voltage that does not exceed a predetermined threshold electric field (1ms23V for the liquid crystal used here) is applied, and the application of the pulse voltage causes the liquid crystal molecules to greatly change their alignment direction from a stable state to a metastable state. (overshoot), but in this case, the applied pulse voltage does not exceed the threshold, so
It immediately returns to its original stable state.

Figure 2b shows the case where a pulse voltage higher than a predetermined threshold voltage is applied, and the liquid crystal molecules overshoot from a stable state to a metastable state, and then the optical Showing response.

Figure 2c shows an enlarged scale of the horizontal axis of Figure 2b (20 msec/div.), in which a transition from a stable state to a quasi-stable state is achieved by applying a pulse voltage greater than a predetermined threshold. It shows the process of returning to a stable state again after a reverberation time (decay time) of approximately 80 ms. The mechanism of this transition from a metastable state to a stable state is essentially different from the case where the threshold value shown in Figure 2a is not exceeded. By doing so, it can be considered that the domain is returned to the original domain again.

In addition to the above-mentioned method of shifting the rubbing directions of the upper and lower substrates, alignment control methods for obtaining such monostable ferroelectric liquid crystal elements include, for example, using a glass substrate coated with ITO as the upper substrate, and This can also be achieved by applying an asymmetrical alignment process to the upper and lower substrates, such as applying a polymer film only to the substrate and applying a rubbing process. FIG. 3 is a plan view schematically showing an example of a sandwiched matrix electrode structure. In FIG. 3, matrix electrode 31
is composed of a scanning electrode group 32 and a signal electrode group 33. FIG. 4 is a voltage-time chart showing an example of the electric signal applied to each electrode S ₁ , S ₂ , S ₃ . . . of the scanning electrode group 32 and the electrode I _ON selected from the signal electrode group 33. .

When displaying a moving image using the electrodes described above, the scanning electrode group 32 is sequentially and periodically selected. Also,
As shown in FIG. 4, the predetermined voltage application time t ₀
On the other hand, if the threshold voltage that brings a monostable liquid crystal cell into a metastable state is V _th , and the reverberation time from the metastable state to the stable state is τ _d , then the electrode signal given to the scanning electrode has a phase (time ) has a voltage V _s at t ₀ and is sequentially selected with a period T. On the other hand, the signal I _ON applied to the selected signal electrode has the same phase (time) t ₀ as the periodic scanning signal and is at voltages V _I and -V _I , and the unselected signal electrodes are in the ground state. . Each of the above items is set to a desired value that satisfies the following relationship.

V _S , V _I < V _th < V _S + V _I T < τ _d In this example, the period T is the metastable state of the liquid crystal, and the amount of transmitted light is the amount of transmitted light immediately after the transition to the stable state. We set the time to reach approximately 10% of the light intensity.

In principle _, a ferroelectric liquid crystal with a monostable state can be driven with only a single-polarity electric signal _, but in this embodiment, as shown in FIG. It is supposed to be applied to the electrode. The reason is that when the number of scanning lines is extremely large,
In a situation where a selected information signal is continuously applied to one signal electrode, by continuously applying pulses of the same polarity, the metastable state can be brought out before the end of scanning one screen. This is to prevent a pixel that has transitioned to a stable state from reverting back to a quasi-stable state.

By applying such an electrical signal, a stable state of the liquid crystal corresponding to an optical "dark" state,
The important effect is that a state change between metastable states corresponding to the "bright" state can be rapidly caused.

Also, the transition time from the metastable state to the stable state τ _d
Since T depends on the liquid crystal material and the operating temperature, the writing time T for one screen can be changed relatively freely.

Therefore, by appropriately selecting the above-mentioned conditions, it is also possible to display a television moving image by, for example, making the writing time T match the cycle of the television signal.

[Effects of the Invention] As explained above, according to the present invention, by using a ferroelectric liquid crystal that has a stable state and a metastable state, display can be performed even with an extremely simple driving method, and It is possible to provide a novel liquid crystal element and its driving method that also enables high-speed display suitable for moving images such as.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of an example of a liquid crystal element used in the present invention, and FIGS. 2 a to 2 c are voltage waveform photographs of an oscilloscope showing an example of the optical response of the embodiment of the present invention.
FIG. 3 is a plan view of the matrix electrode used in the present invention, and FIG. 4 is a voltage waveform diagram of the electrode. 11; Substrate coated with transparent electrode, 12; Liquid crystal molecule layer, 13; Liquid crystal molecule, 14; Dipole moment (P⊥), 24; Upward dipole moment,
24'; Downward dipole moment, 23; First stable state, 23'; Second stable state, 31; Matrix electrode, 32, S ₁ , S ₂ , ...; Scanning electrode, 3
3, I ₁ , I ₂ , ...; signal electrode.

Claims (1)

[Scope of Claims] 1 a. A matrix electrode that uses a plurality of pixels formed at the intersections of a scanning electrode group and a signal electrode group in a display screen, and is arranged between the scanning electrode group and the signal electrode group, and 1 It has a ferroelectric liquid crystal that generates two states: one stable state and one metastable state that appears when an electric field exceeding a threshold is applied and transitions relaxedly to the one stable state when no electric field is applied. After applying an electric field exceeding the threshold to the liquid crystal cell, and b to the selected pixel, the liquid crystal molecules are transferred from a stable state to a metastable state, and the relaxation time for returning from the metastable state to the stable state is determined by the scanning A liquid crystal device comprising means for applying a voltage to the scanning electrode group and the signal electrode group so that the time required to scan the electrode group is longer than the time required to form one screen.