Liquid crystal cells – elements and systems – With specified nonchemical characteristic of liquid crystal... – Within smectic phase
Reexamination Certificate
2000-07-14
2003-06-03
Kim, Robert H. (Department: 2871)
Liquid crystal cells, elements and systems
With specified nonchemical characteristic of liquid crystal...
Within smectic phase
C349S174000, C345S204000, C345S089000
Reexamination Certificate
active
06573970
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display device for use in devices such as display apparatus for displaying text and figures and the like, dimmer devices by which the degree of transmittance of incident light is varied, and optical shutters.
2. Description of the Related Art
A display device which utilizes the optical switching phenomenon of a ferroelectric liquid crystal (FLC) has been proposed by N. A. Clark and S. T. Lagerwell (App. Phys. Lett., Vol. 36, p899 (1980)) as a liquid crystal display which can offer a wide viewing angle and a fast response. However, the display device (a surface stabilized ferroelectric liquid crystal (SSFLC) optical device) is bistable, and suffers from the problem that generating a gradation display by controlling the voltage is difficult. To overcome this problem, several methods have been proposed for using an FLC to realize a gradation display. One such example is Japanese Unexamined Patent Application, First Publication No. Hei 6-194693, in which a technique for adding fine particles to a liquid crystal is disclosed. By distributing the fine particles uniformly though the liquid crystal, a dielectric constant distribution is formed, and as a result, a distribution is formed for the effective voltage applied to the liquid crystal. The effective voltage distribution makes a gradation display possible. Furthermore, Japanese Unexamined Patent Application, First Publication No. Hei 9-236830 a technique is disclosed for forming a striped structure within a liquid crystal by reaction with a monofunctional monomer. By providing threshold characteristics which differ for different local regions within the liquid crystal, the striped structure controls the domain surface area generated on voltage application, and therefore makes a gradation display possible. However the aforementioned techniques suffer from problems such as the generation of orientation defects, which is linked to a reduction in contrast resulting from the introduction of foreign material into the liquid crystal, and the suppression of increases in driving voltage.
An alternative liquid crystal material which has an antiferroelectric phase has been reported by Chandani et al. (Jpn. J. Appl. Phys., 28 (1989), L1265), and a display device utilizing an antiferroelectric liquid crystal (AFLC) has also been proposed (Jpn. J. Appl. Phys., 29 (1990), 1757). AFLC materials have tristability based on a phase transition between an antiferroelectric phase and a ferroelectric phase, and by switching between phases under a bias voltage application, a display device can be manufactured in which gradation display can be achieved by voltage control. However, this type of device also suffers from problems such as the necessity for a bias voltage to achieve a gradation display, and the fact that the driving waveform for a high precision display device with a large number of scanning lines is very complex.
In contrast, Inui et al. and Tanaka et al. have reported an AFLC material (hereafter termed a non-threshold AFLC material) for which the curve displaying optical transmittance relative to applied voltage is a V shape (Proceedings of the 21
st
Liquid Crystal Symposium 2C04, p.222 (1995), and p.250 (1995)). This V shaped characteristic refers to a property wherein, as shown in
FIG. 1
, application of a positive voltage produces a continuous variation in the transmittance, and application of a negative voltage also produces a continuous variation in the transmittance with the shape of the curve being substantially symmetrical with the curve representing application of a positive voltage across an axis at a voltage of zero volts. A non-threshold AFLC device utilizing this type of material is reported as having no clear threshold for the phase transition, and displaying a small hysteresis characteristic.
Moreover, Takei et al. have reported a 5.5 inch diagonal liquid crystal optical device which combines the above non-threshold AFLC device with a thin film transistor (TFT) (papers presented at the 74
th
Workshop of The Japan Society for the Promotion of Science, “142
nd
Committee of Organic Materials for Information Display” ,Section A (liquid crystal materials), p14, 1999).
Typically, non-threshold AFLC materials display high spontaneous polarization values of 100 (nC/cm
2
) or more, and in the aforementioned liquid crystal optical device which combines a non-threshold AFLC device with a TFT (papers presented at the 74
th
Workshop of The Japan Society for the Promotion of Science, “142
nd
Committee of Organic Materials for Information Display” , Section A (liquid crystal materials), p14, 1999), a non-threshold AFLC material with a spontaneous polarization of 229 (nC/cm
2
) was used. In order to drive this type of non-threshold AFLC material with a high spontaneous polarization value, an electrical charge in proportion to the spontaneous polarization needs to be injected. However, a problem arises in that because there is a limit on the amount of charge that can be supplied from a TFT, charge injections which span several frames become necessary, which slows down the screen display on the liquid crystal display device.
One example of a method for resolving the above problem comprises the addition of a large auxiliary capacitance to a TFT. However, with a large auxiliary capacitance, the numerical aperture of the liquid crystal optical device decreases, meaning the display will darken. Moreover, investigations by the inventors have revealed that as the auxiliary capacitance is increased the RC time constant also increases, and that in order to carry out sufficient writing within a predetermined writing time period, the on-state resistance of the TFT must be lowered and the TFT characteristics improved. Hence, because the capacity value increases, if the on-state current of the TFT can not be sufficiently ensured, then the writing may not be completed within the writing time period. Consequently, if the TFT characteristics are determined, and the spontaneous polarization value of the liquid crystal material and the panel structure are also determined, then a threshold value will exist for the optimum auxiliary capacitance, and an auxiliary capacitance in excess of this threshold value will cause an increase in the RC time constant, a reduction in the injected charge within the writing time period, and as a result a reduction in the charge for writing to the liquid crystal.
In order to enable the driving of a liquid crystal with a high spontaneous polarization, either a TFT with the required characteristics can be used, or alternatively writing can be conducted at a high voltage. However, in such cases the following types of problems arise. Firstly, there is a need to develop new TFTs with suitable characteristics. Secondly, drive circuits which enable the application of a high driving voltage also need to be developed. Even if these two criteria are met, a large charge still needs to be used in order to drive a liquid crystal with a high spontaneous polarization, and consequently the power consumption will be extremely large.
The various problems described above resulting from high spontaneous polarization values are not limited to gradation display devices which use non-threshold AFLC materials, and are also an issue for gradation display devices which use an aforementioned FLC, and AFLC gradation display devices which utilize tristability.
In addition, for AFLC gradation display devices which utilize tristability, it has been shown that the spontaneous polarization needs to be lowered in order to minimize the hysteresis distortion. For example in Japanese Unexamined Patent Application, First Publication No. Hei 10-279534, in order to reduce the hysteresis distortion for an AFLC display device which utilizes tristability, the spontaneous polarization was reduced by using an antiferroelectric composition which incorporates a new racemic compound. However, the spontaneous polarization value using this method was reported in a worki
Saitoh Goroh
Takatori Ken-ichi
Duong Thoi
NEC Corporation
Young & Thompson
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