JP2628156B2 - Ferroelectric liquid crystal electro-optical device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION The present invention relates to a method for driving a device that performs electro-optical conversion using spontaneous polarization and negative dielectric anisotropy of a ferroelectric liquid crystal.

[Summary of the Invention]

The present invention utilizes a spontaneous polarization of a ferroelectric liquid crystal to selectively align liquid crystal molecules in a first stable state or a second stable state, and utilizes the negative dielectric anisotropy of a ferroelectric liquid crystal. In the electro-optical conversion device that holds each stable state, immediately after alignment in the first stable state or the second stable state, a high-frequency electric field is applied to bring the molecules of the ferroelectric liquid crystal into contact with the glass substrate. It was held in a horizontal position.

[Conventional technology]

2. Description of the Related Art An electro-optical converter using spontaneous polarization and negative dielectric anisotropy of a ferroelectric liquid crystal has been known. For example, it is disclosed in JP-A-60-176097.

FIG. 2 is a perspective view of a conventional ferroelectric liquid crystal cell (hereinafter, simply referred to as a liquid crystal cell). 1-1 is a pair of transparent glass substrates arranged opposite to each other. Reference numeral 2-2 denotes a uniaxial horizontal alignment film disposed on the inner plane of the substrate 1, for example, a rubbing film of polyimide is used. The rubbing directions of the pair of alignment films are substantially parallel. Reference numeral 3 denotes a ferroelectric liquid crystal, for example, a chiral smectic liquid crystal (hereinafter, referred to as SmC ^* ), which has spontaneous polarization in a direction orthogonal to the major axis of the liquid crystal molecules (hereinafter, referred to as molecular axis). In this case, a material having a negative dielectric anisotropy Δε at least over a certain frequency is selected. When Δε <0, it means that induced polarization occurs in a direction perpendicular to the molecular axis due to an external electric field in a constant frequency region. Thus, when an electric field is applied, the molecules move perpendicular to the electric field.

The SmC ^* molecules are sandwiched between the substrates 1-1 and the alignment film 2
Due to the influence of -2, a horizontal orientation as shown in the figure is formed and a layer is formed. When this is observed from above, the liquid crystal molecules are divided into two domains. In the first domain, the molecular axis is inclined by + θ with respect to the normal 4 of the layer. This is the first stable state 5. The spontaneous polarization 7 of the liquid crystal molecules points upward. In the second domain, the molecular axis is relative to the layer normal 4,
−θ inclined. This is the second stable state 6. Spontaneous polarization 7 points downward. By utilizing the fact that the directions of the spontaneous polarization 7 are opposite to each other in the both stable states, one of the bistable states is selected by positive and negative DC pulses. 8-8
Is a pair of polarizing plates arranged to face each other with their polarization axes orthogonal to each other, and optically distinguishes between a first stable state and a second stable state by birefringence. For example, the first stable state is converted to a light blocking state (hereinafter, referred to as black), and the second stable state is converted to a light passing state (hereinafter, referred to as white). 9 and
Reference numeral 10 denotes a matrix electrode for applying a drive voltage to the SmC ^* thin film 2, and as shown in FIG. 3, reference numeral 9 denotes a scanning electrode (hereinafter, referred to as a common), and reference numeral 10 denotes a signal electrode (hereinafter, referred to as a segment).

FIG. 4A shows a conventional driving waveform applied to one matrix pixel (hereinafter, referred to as a dot) in line-sequential driving using the AC bias averaging method. Pulses P ₁ and P ₂ of the positive and negative (common 9 reference) having a peak value of more than the selected time period threshold value in the first frame is added sequentially. Wherein P ₁ and P ₂ is a pulse width, the absolute value of the peak values are equal. Liquid crystal molecules by the positive pulse P ₁ is aligned to the first stable state and switched by the negative pulse P ₂ to Tsuzuku aligned to the second stable state. This state is maintained by the application of the AC pulse trains P ₃ and P ₄ during the non-selection period. This is because the peak values of the AC pulse trains P ₃ and P ₄ are lower than the threshold voltage. Therefore, in the first frame, the first
Is written in the stable state. Subsequently, in the second frame, white is written because the polarity of the pulse is reversed.

However, since P ₅ and P ₆ is a threshold voltage below the white is not written, black written in the first frame is held. Here, the pulse width of P ₃ and P ₄ and P ₅ and P _6, the absolute value of the peak values are equal.

[Problems to be solved by the invention]

However, if it is conventional driving method, from application of P ₂ pulse, transmitted light characteristic of FIG. 4 have an amplitude ΔI as shown in (b) the liquid crystal cell changes. The locus of the center value of ΔI is indicated by a dashed line. That is, the center value of ΔI gradually increases. This is the pulse P ₂ above the threshold voltage
It has been found that this is due to a relaxation phenomenon from a state where the liquid crystal molecules are aligned in one stable state to a more stable state determined by the elastic constants of the alignment film and the liquid crystal.
That is, one stable state includes a stable state generated by a pulse having a threshold voltage or more (hereinafter, this state is referred to as a stable state A).
There are two types of stable states (hereinafter referred to as stable states B) generated by a pulse having a threshold voltage or less. FIG. 5 shows the state of the liquid crystal molecules in these two stable states.

The long axis of the liquid crystal molecules 11 is oriented in a direction perpendicular to the paper surface,
The circle in the figure represents the cone of the liquid crystal molecule 11.

FIG. 5A shows a stable state A, and FIG. 5B shows a stable state B. That is, the liquid crystal molecules 11 in the stable state A are completely horizontal with respect to the glass substrate. However, the liquid crystal molecules 11 in the stable state B are not completely horizontal with respect to the glass substrate, but are inclined in a direction perpendicular to the substrate. Comparing the contrast in these two states shows that the stable state A is better.

Therefore, there is a problem that a sufficient contrast cannot be obtained by the conventional driving method.

[Means for solving the problem]

An object of the present invention is to solve the above-mentioned problems of the prior art, and specifically, to devise a drive waveform so as not to cause the relaxation phenomenon. Therefore, the pulse P ₃ or P ₄ in the pulse P ₂ or P ₆ and the non-selection period in the selection period
After applying, by applying a high AC pulse frequency than the pulse P ₁ to P _6, by eliminating the relaxation phenomenon, with improved contrast.

FIG. 1A shows a drive waveform applied to a dot according to the present invention. Similarly, (b) shows a change in transmitted light intensity.
In the first frame, the pulses P ₁ and P ₂ have the same function as in the conventional example. As applied to P _1, a short pulse train P ₇ than the pulse width of P ₂ immediately. The peak value of the pulse train P ₇ are the same as pulse P _3, P _4. Thereafter, a pair of bipolar pulses of pulses P ₃ and P ₄ are applied. This works the same as the conventional example. pulse
This time immediately after the P _4, the pulse train P ₇ and frequency but the same peak value is applied to the same pulse train P ₈ and pulse P _1, P _2. Thereafter, the pulse train P ₈ is applied immediately after the pair of bipolar pulses of the pulse P _3, P ₄ is applied. In the second frame, the pulse train P ₇ immediately after the pulses P ₅ and P ₆ having the threshold voltage or less are applied.
There is applied, the pulse train P ₈ is applied immediately after the pair of bipolar pulses of the pulse P _3, P ₄ is applied.

Next, a change in transmitted light intensity when the above-mentioned waveform is applied will be described with reference to FIG. The transmitted light greatly changes due to the pulses P ₁ and P ₂ and is written in the first stable state determined by the polarity of the pulse P ₂ . That is, it becomes black.

Therefore, in the figure, the level W indicates white and the level B indicates black. This black level corresponds to the aforementioned stable state A. Then the transmitted light intensity and applying a pulse train P ₇ starts to shift to a stable state B by relaxation. Because
The pulse width of the pulse train P ₇ is because the spontaneous polarization of the liquid crystal molecules is chosen short enough not to respond, by substantially pulse train P ₇ because the liquid crystal molecules are not affected at all. Next, the transmitted light intensity changes with the amplitude ΔI by the pulses P ₃ and P ₄ . This is the same as the conventional example. However, the pulse train P ₈
Transmitted light intensity to be applied immediately after the P ₄ returns to a stable state A. The reason is as follows. Similar pulse width of the pulse train P ₈ and P _7, spontaneous polarization is chosen short enough not to respond. Therefore, the liquid crystal molecules do not move due to spontaneous polarization. However, since the dielectric anisotropy is negative in this frequency range, when a voltage pulse is applied, a force acts so that the liquid crystal molecules become horizontal to the glass substrate. (Hereinafter, this force is referred to as dielectric torque.) It has been found that the dielectric torque is proportional to the electric field strength and time. Thus, by applying a high voltage pulse, such as pulse train P _8, the liquid crystal molecules is continuously applied until the glass substrate and the horizontal or steady state A.

Incidentally, the pulse sequence P ₇ described above has a low peak value, rather than the dielectric torque is sufficient, it is not relaxation to a stable state B occurs. Thereafter, a similar pulse train is repeated,
This is the same for the frame of. That is, immediately after the application of the pulses P ₃ and P ₄ , the above-described relaxation phenomenon starts to occur, so that if left as it is, the molecules move to the stable state B. However, the molecules are held in the stable state A by the dielectric torque in order to immediately apply the high-frequency, high-peak pulse train for an appropriate time. Therefore, it is possible to realize the contrast in the almost stable state A.

Although the present invention has been described only with respect to the driving waveform shown in FIG. 4 (a), the present invention can be applied to a driving method in which writing and erasing are performed using a pair of bipolar pulses in principle.

〔Example〕

A specific circuit for generating the driving waveform shown in FIG. 1A will be described. FIG. 6 shows a common drive circuit, and FIG. 7 shows a segment drive circuit. For the most part, it is the same as a conventional twisted nematic drive circuit. The difference is that the gate groups 12, 13 and each shift register 14 have an INV input.

FIG. 8 shows a time chart of a signal input from the outside. The FLM signal (common data signal) is frequency-divided by a JK flip-flop to become a signal F. Further, an exclusive OR operation with the DF signal (alternating signal) is performed to obtain a signal C. Thus, the transmission gate group 15 of the common and segment drive circuits is opened and closed. The truth value of opening and closing the transmission gate group 15 is the same as the conventional one. In the segment driving circuit, the signal F is exclusive-ORed with the DATA signal and input to the shift register 14. Also, IN
The V signal is input to each shift register 14.

The fact that the phase of the signal C is changed between the first frame and the second frame is the reason that the phases of the first frame and the second frame are reversed in FIG. 1A. Also, since the DATA signal is exclusive ORed with the signal F, FIG.
The peak values of P ₁ and P ₂ are different from those of P ₄ and P ₅ . That is, only dots that should be black in the first frame are written,
Only the dots that should be white in the frame are written.
Therefore, one screen writing is performed in two frames. or,
One cycle of CL1 signal (common shift clock signal) is DF
It is a signal repetition pattern. The last part of the repeating pattern of the DF signal, i.e. during the last third period of CL1 is frequency is high, which is generating a pulse train P _7, P _8. However, P _7, P _8, such as FIG. 1 in this state (a) is not output. This is done by the INV signal. The INV signal is at a high level in a portion where the frequency of the DF signal is high. With the rise of the High level, the latch 16 of the segment drive circuit is set to all the set states, and the data of the shift register 14 of the common drive circuit is all inverted. In this way, pulse trains P ₇ and P ₈ can be obtained. In FIG. 8, the time of the high frequency part in the DF signal is written so that the period of the CL1 becomes 1/3. And is not necessarily 1/3.

[Effects of the Invention] As described above, according to the present invention, two optical states of white and black are written by line-sequential matrix driving using spontaneous polarization and negative dielectric anisotropy of SmC ^* molecules. In the electro-optical device, immediately after applying one bipolar pulse composed of pulses having the same peak value and the opposite polarity, a bipolar pulse having a frequency higher than that of the bipolar pulse is applied to the entire matrix of the matrix for an appropriate time. The application to the dots has the effect that an electro-optical device with high contrast can be obtained.

[Brief description of the drawings]

FIG. 1A is a waveform diagram applied to a matrix dot of the present invention, FIG. 1B is a diagram showing transmitted light characteristics at that time,
FIG. 2 is a perspective view of a conventional liquid crystal cell, FIG. 3 is a diagram showing electrode arrangement of the conventional liquid crystal cell, FIG. 4 (a) is a driving waveform diagram of the conventional liquid crystal cell, and FIG. 5 (a) and 5 (b) are cross-sectional views of a liquid crystal cell, FIG. 6 is a common drive circuit diagram of the present invention, FIG. 7 is a segment drive circuit diagram, and FIG. It is a chart figure. 1,1 ... substrate 2,2 ... alignment film 3 ... SmC ^* thin film 8,8 ... polarizer 9,10 ... electrode 11 ... liquid crystal molecules 12,13 ... gate group 14 ... shift register 15 …… Transmission gates 16 …… Latch

Claims (3)

(57) [Claims] 1. A ferroelectric liquid crystal thin film having spontaneous polarization in a direction perpendicular to the major axis of liquid crystal molecules and having a negative dielectric anisotropy at least in a high frequency region, and a glass substrate sandwiching the thin film. An alignment film for forming uniform domains of liquid crystal molecules, a pair of electrodes for applying a voltage to the thin film, and a pulse width and a peak value for aligning the liquid crystal molecules in a first stable state or a second stable state. A driving circuit for supplying a waveform composed of at least a pulse composed of pulses having the same absolute value and opposite polarities to the picture element, and optically controlling the first and second stable states of the liquid crystal molecules. In the ferroelectric liquid crystal electro-optical device comprising a converting member for distinguishing, the pulse waveform supplied to the picture element is immediately after the bipolar pulse having a high selection period is applied, and the bipolar pulse having a low non-selection period is applied. Sex pulse In the period leading up to the pressure, the peak value is the same as the peak value of the low bipolar pulse of the non-selection period,
Immediately after the bipolar pulse train having a frequency higher than the frequency of the bipolar pulse is applied and the bipolar pulse having a low non-selection period is applied, the bipolar pulse having a low non-selection period is applied. In the period until the peak value is higher than the peak value of the low-polarity pulse of the non-selection period, a bipolar pulse train having a frequency higher than the frequency of the bipolar pulse is applied. Ferroelectric liquid crystal electro-optical device. 2. The frequency of a bipolar pulse having a higher frequency than the one bipolar pulse is set in a frequency band in which the ferroelectric liquid crystal thin film exhibits negative dielectric anisotropy. The ferroelectric liquid crystal electro-optical device according to claim 1. 3. The application time of the bipolar pulse having a frequency higher than that of the one bipolar pulse is set to be equal to or longer than the minimum time during which the liquid crystal molecules can be almost or completely horizontal with the glass substrate. The ferroelectric liquid crystal electro-optical device according to claim 1, wherein: