WO2011161663A1

WO2011161663A1 - Liquid crystal display devices

Info

Publication number: WO2011161663A1
Application number: PCT/IE2011/000032
Authority: WO
Inventors: Vij. K. Jagdish; Panarin Yuri; Nagaraj Mamatha
Original assignee: The Provost, Fellows, Foundation Scholars, And The Other Members Of Board, Of The College Of The Holy And Undivided Trinity Of Queen Elizabeth, Near Dublin
Priority date: 2010-06-25
Filing date: 2011-06-24
Publication date: 2011-12-29

Abstract

A liquid crystal display device comprises first and second substrates placed between cross polarisers. Homeotropically aligned orthogonal biaxial smectic liquid crystal molecules are located between the substrates. An electrode drive is arranged on the substrates to cause in-plane switching of the direction of the short axes. The electrode drive comprises a first pair of electrodes which are arranged to generate a first in-plane electric field which is oriented parallel relative to the transmission axis of one of the polarisers, and a second pair of electrodes which are arranged to generate a second in-plane electric field which is oriented so as to deviate from the transmission axis of one of the polarisers. The two pairs of electrodes allow active switching from the bright state to the dark state. The liquid crystal display has a higher contrast ratio and considerably faster switching time than conventional liquid crystal displays.

Description

'LIQUID CRYSTAL DISPLAY DEVICES'

Introduction

Liquid crystal displays (LCDs) are widely used in large-size flat-panel TVs and PC monitors.

The modes most widely used for TV application are an in-plane switching (IPS) mode (Ref 1 , 2) and a vertical alignment (VA) mode (Ref 3, 4). LCDs using nematic liquid crystals (LCs) have a relatively slow response, and thus ferroelectric LC (FLC) (Ref 5) or antiferroelectric LC (AFLC) (Ref 6) modes are required. To display continuous grey levels, which cannot be realised in conventional FLC and AFLC displays, V-shaped switching (VS) mode (Ref 7, 8) has been proposed.

In spite of the success of Liquid Crystals as working media for modern flat panel TV and computer monitors, there is still a set of problems and limitations, which need to be addressed in a different way from traditional technologies. Conventional Liquid Crystals are Nematic Liquid Crystals (NLC). One of the main problems in further improvement of LCD technology is a limited switching speed, which is not sufficiently fast for new LCD modes with time sequential colour division and strobing LED backlight, which needs at least three times faster switching than in modern LCD modes such as spatial division colour mode. Possible solutions are to use dual-frequency control [9] or Ferroelectric Liquid Crystals (FLC) [10]. However these options suffer other problems: The dual-frequency control mode suffers from high power consumptions at high frequency due to the pixel capacitance. The FLC displays possess a relatively low contrast ratio and are very sensitive to mechanical vibration. Therefore they have not been commercialised for large-scale production of displays and other devices.

It has been proposed [1 1] to use biaxial liquid crystals as working media and to electrically control the secondary director m with fixed primary director n , and a homeotropically aligned LC layer. One known approach [12] utilises the cooperative motion of bent molecules with quasi long- range order of dipoles in a macroscopically uniaxial SmAP_R phase.

Another known approach [13] utilises a macroscopically uniaxial nematic phase consisting of microscopical biaxial and polar cybotactic clusters (domains). In the absence of an electric field both of these approaches are macroscopically uniaxial, appearing as a dark state between the crossed polarizers. An application of electric field aligns the short molecular axes and induces a macroscopic biaxiality performing a bright state. There is a need for new LCD modes which will have a fast response, high contrast ratio, wide viewing angle, and continuous grey levels.

Statements of Invention

According to the invention there is provided a liquid crystal display device comprising:- first and second substrates placed between cross polarisers, between said substrates, homeotropically aligned orthogonal biaxial smectic liquid crystal molecules having long axes and short axes, and an electrode drive arranged to cause in-plane switching of the direction of the short axes the electrode drive comprising: a first pair of electrodes which are arranged to generate a first in-plane electric field which is oriented parallel relative to the transmission axis of one of the polarisers; and a second pair of electrodes which are arranged to generate a second in-plane electric field which is oriented so as to deviate from the transmission axis of one of the polarisers.

The two pairs of electrodes allow active switching from the bright state to the dark state. In the dark state all the short axes of the liquid crystal molecules within each smectic layer plane are homogeneously oriented parallel to one of the polarisers. This dark state results in a higher contrast ratio than in conventional liquid crystal displays. In addition, the switching time is in the sub-milliseconds range, considerably faster than in conventional liquid crystal displays based on nematic liquid crystals. According to the invention there is also provided a liquid crystal display device comprising:- first and second substrates placed between cross polarisers, between said substrates, homeotropically aligned antiferroelectric smectic liquid crystal molecules having long axes and short axes, and an electrode drive arranged on the substrates to cause in-plane switching of the direction of the short axes.

The long axes may be perpendicular to the substrates.

The short axes may be in the smectic layer plane which is parallel to the substrates in this geometry.

The switching is preferably about the long axes.

In one case the electrode drive is arranged on a single substrate. In one embodiment the electrode drive is adapted to change the short axis direction by up to about 45°.

In another case the electrode drive comprises electrodes on the first substrate arranged to cause switching to one short axis direction and electrodes on the second substrate arranged to cause switching to another short axis direction. Electrodes on one substrate may be arranged substantially parallel to a polarizer, and those on the other substrate are arranged at an angle to the polarizer. Said angle may be up to about 45°.

We have demonstrated a new LCD mode using a biaxial smectic A phase of banana-shaped liquid crystal molecules.

The invention provides fast electrooptical switching in a new type of Liquid Crystals, in particular for use in LCD technologies, new LCD modes, such as sequential-color, LED backlight, and the like. The liquid-crystal display cell of the invention contains a homeotropically aligned biaxial orthogonal smectic liquid-crystal either antiferroelectric (SmAP_A) or ferroelectric (SmAP_F) comprising non-tilted bent-core molecules. There may be two pairs of side electrodes placed between the crossed polarizers. An application of an electrical field orients the direction of the short molecular axes (secondary director m ) in the direction of electric field. A change in the direction of the electric field causes the in-plane switching (IPS) of the optical axis from parallel to the polarizer axis (dark state) to the direction at (ideally) 45 degrees (bright state) and back to the initial state. The switching time is in the sub-milliseconds range, considerably faster than in conventional LCD based on nematic Liquid Crystals, and can be used in new sequential color LCD modes.

The working media used in the invention is a LC layer of (macroscopically) biaxial SmAP_A phase. The sample is biaxial even in the absence of an electric field. There is no need to induce the biaxiality, which requires a rather high electric field. A moderate electric field is sufficient to rotate the short molecular axes (secondary director m ).

The position of the primary director is fixed by homeotropically aligned smectic layers preventing unwanted switching or rotation of the long molecular axes. In the invention an electric field is typically always on. The electric field is applied to one of two electrodes or set of electrodes (dark/bright states) or gradually distributed between them enabling a grey-scale.

This invention has a potential application for new LCD technologies with electronic control similar to IPS modes but offers considerably faster switching which is suitable for sequential color display modes with LED backlight, and the like.

Brief Description of the Drawings

The invention will be more clearly understood from the following description of an embodiment thereof, given by way of example only, with reference to the accompanying drawings, in which:

Fig. l Molecular structure of SmAP_A material - C -64 - (Reference 14), Fig. 2 are diagrams illustrating (a) primary and secondary directors, (b) switching around primary director;

Fig. 3 Molecular structure of biaxial phases comprising: (a) board-like molecules - SmA_b; (b) bent-core molecules - SmAP_A (c) Electric field induced transition to ferroelectric state;

Fig. 4 Structure of a proposed new mode LCD cell, (a) dark state- the electric field is applied to bottom pair of electrodes and (b) bright state- the electric field is applied to top pair of electrodes;

Fig. 5 (a) Schlieren texture in SmAP_A phase in the absence of electric field, (b) and (c) are textures in two (dark and bright) states under E= 1 V/μπι, 1 10 Hz and T= 1 10

°C;

Fig. 6 Response time as a function of Electric field strength at a frequency of 1 1 OHz (■) and Response time as a function of Frequency at a field of 1 V/μπι, (·). Both plots are at a temperature of T = 1 10 °C;

Fig. 7 is a diagram illustrating one configuration of electrodes;

Fig. 8 is a diagram illustrating another configuration of electrodes;

Fig. 9 is another diagram illustrating a configuration of electrodes;

Fig. 10 is a diagram illustrating the relationship of the directions of the electric field to the polariser axes; and

Fig. 1 1 Biaxiality in SmAP_A phase vs. in-plane electric field (of frequency 1 10Hz) at T=1 10°C in an 8.7μπι homeotropic cell. Insets: Textures under crossed polarizers for different electric field strengths. Field direction is at an angle of 45° to the polarizer / analyzer. Distance between the electrodes is 180μπι.

Detailed Description Nematic LC is the simplest phase used in display device technology. This is an optically uniaxial nematic (Nu or N) phase, in which the constituent rodlike molecules, on average, orient about a common axis called the director n . Biaxial nematic phase (Nb) are formed by bent- shape molecules (Fig. 1 ). In such phases there is, additionally, a correlation of the molecules in a direction m perpendicular to the primary director n (Fig. 2a). In the biaxial nematic phase, fast switching between different birefringent states may be possible by the rotation of the short axis m due to the dielectric anisotropy (ε,γ,-ει) while the orientation of the long axis n is fixed as shown in the Fig. 2b. However, practically it is difficult to realise an independent switching of short axis m with fixed long axis Π and it is usually accompanied by unwanted rotation of the primary director as well, as discussed in Ref. [13].

The invention utilises a biaxial smectic A (SmAb) phase in which the molecules are packed in so-called smectic layers wherein the primary director n is along the layer normal due to the alignment on the substate and the containment of the molecules within the smectic layers, but have an additional director m in the plane of the layers, whose symmetry depends on the type of constituent molecules: non-polar D₂ii if the molecules are board-like (Fig. 3a) and polar C_2v if they are bent-core or banana-shaped molecules (Fig. 3b). In the latter case the smectic layer possesses spontaneous polarization Ps parallel (or anti-parallel) to the secondary director m . The secondary directors in the neighbouring smectic layers are anti-parallel to each other forming the antiferroelectric phase SmAPA, where P represents polar ordering within a smectic layer and subscript A represents antiferroelectric packing between the adjacent layers. Under an application of an electric field higher than a threshold level the antiferroelectric state transitions to the optically equivalent field-induced ferroelectric state, SmAPp. Recently (16) new orthogonal biaxial ferroelectric LC phase SmAPF was discovered. In this phase the spontaneous polarizations Ps in the neighboring layers are parallel to each other even in the absence of applied electric field. Such an LC material can also be used in proposed device geometry. The liquid-crystal display cell contains a homeotropically aligned orthogonal biaxial smectic liquid-crystal (SmAPA or SmAPp) comprising non-tilted bent-core molecules such that the smectic layers are parallel to the glass plates placed between the crossed polarizers as shown in Fig. 4. The electrodes are arranged in such a way as to enable the switching of the short axes in the plane of the smectic layers in the direction of the electric field by (ideally) 45 degrees from each other. In one embodiment of the invention this is achieved by using two pairs of electrodes. One of these (bottom) pairs is parallel (or perpendicular) to the analyzer or polarizer (Fig. 4a) and the other (top) pair is at 45 degrees to the analyzer (polarizer) (Fig. 4b). Initially, in the absence of applied field there is a typical Schlieren texture [Fig. 5 (a)] confirming the biaxial nature of the S IAP_A phase. Application of an electrical field to the bottom pair of electrodes orients the direction of the short molecular axes in the direction of the electric field and therefore parallel to the polarizer as shown in the Fig. 4 (a). This corresponds to the dark state shown in the Fig. 5 (b). An application of an electrical field to the top pair of electrodes orients the short molecular axes (secondary director tn ) at 45 degrees to the analyzer (polarizer) as shown in the Fig. 4 (b) realising a bright state shown in the Fig. 5 (c). For the LC sample used the optical biaxiality was about δη ~ 0.016 therefore the optimal cell thickness is about 17- 18 μηι. Simultaneous application of an appropriate electric field of different values to both electrodes pairs will produce an analog grey scale.

Fig. 6 shows the measured response time (for the above cell configuration) as a function of the electric field at a frequency of 1 10 Hz and as a function of frequency at a fixed electric field of 1 V/um. The fastest response we obtain for this sample is 400 us for fields greater than 1.7V7 im. Table 1 lists the possible display modes in SmAP_R and SmAP_A phases with the display parameters obtained for CK64 material. Since the alignments of long and short axes of the molecule are fixed by the smectic phase, we do not need to use any external treatment like rubbing to obtain a uniform alignment. This provides with the advantage of high contrast ratio for the displays. At a general level, the geometry of the cell and electrodes is broadly similar to the in-plane switching (IPS) mode used for modern flat panel TVs and monitors. However in the cell of the invention the electric field controls the switching of the secondary director instead of the primary one and therefore the switching time of this mode lies in the sub-milliseconds range (as shown in the Fig 6). This is considerably faster than in a conventional LCD based on nematic Liquid Crystals and can be used in sequential color LCD modes. Another difference is that the switching can occur in response to the dielectric anisotropy (at low electric field) or the polar interaction of spontaneous polarization with electric field at higher electric fields. In a laboratory experiment two pairs of electrodes are placed on each (top and bottom) substrates at the angle 45° between them as shown in Fig. 7. When any electric field is applied to the electrodes, the molecules, in a SmAP_A state, rotate around their primary axes n causing their short axes ( m ) to align with the field. However, planar (or in-plane) switching in IPS thin film transistor (TFT) liquid crystal of the electrodes displays can also be realized on one TFT substrate as shown in Figs. 8 and 9.

Referring in particular to Fig. 9 there is illustrated a liquid crystal display device which comprises a first substrate 50 and a second substrate 51 which are arranged between cross polarisers. Between the substrates 50, 51 , a layer 52 of homotropically aligned orthogonal biaxial smectic liquid crystal molecules are arranged. The molecules have long axes and short axes. An electrode drive is arranged to cause in - plane switching of the direction of the short axes of the molecules. The electrode drive comprises a first pair of electrodes 55, 56 and a second pair of electrodes 58, 59. The first pair of electrodes 55, 56 are arranged to generate a first in-plane electric field which is oriented parallel relative to the transmission axis of one of the polarisers. The second pair of electrodes are arranged to generate a second in-plane electric field which is oriented so as to deviate from the transmission axis of one of the polarisers, in this case by about 45°. The first pair of electrodes 55, 56 may be arranged on one of the substrates 50, 51 and the second pair of electrodes 58, 59 may be arranged on the other of the substrates 50, 51 as illustrated in Fig. 7. Alternatively, and as illustrated particularly in Figs. 8 and 9 the first and second pairs of electrodes may be arranged on only one of the substrates. In this case an insulator layer 60 is provided between the electrode pairs.

Regardless of particular configuration of the electrodes, the relationship of the directions of the electric field to the polariser axes (P| and P₂) are shown in Fig. 10. E] (E₂) are the direction of the electric field with first (second) activated electrode (or set of electrodes) and mi (m₂) are the orientations of the secondary director m when Ei (E ) is applied.

The optical transmittance of this system can be expressed as:

T = k^■ sm^{' 2} (2φ) , where k represents constant power and birefringence

d / λ)) terms and φ is the angle between the secondary director m and the polariser (Pi) axis. The contrast ratio of such bright and dark states is 1000: 1 ; this measurement has been limited by the quality of the polarizer. Simultaneous activation of both electrodes (or set of electrodes) with different voltages results in a variation of the direction of total electric field (and therefore secondary director m ) at any direction between Ei and E₂ (0° < φ < 45°), providing a grey scale according to the above equation. The compound C 64 [structure given in Fig. 1 ] exhibits a unique sequence of phases: Cr 37.8 °C SmAP_A 1 1 1 °C SmAP_R 157.5 °C SmA 163.9 °C I¹⁴ with both SmAP_A and SmAP_R phases existing over a broader temperature range. Further detail on electric field induced textural transformations and the relaxation phenomena in SmAP_A phase of this compound are given in reference 15, the entire contents of which are herein incorporated by reference. The material shows negative dielectric anisotropy throughout the liquid crystalline temperature range. Polymer AL60702 (JSR Korea) is used as an alignment agent for the homeotropic cells. The cell spacing is 8.7 μπι, the distance between two stripe 1TO electrodes is ~ 180 μπι. On cooling, the cell with LC in the SmAP_R phase under crossed polarizers, we find that the cell is well aligned and exhibits perfect extinction in the absence of external electric field, whereas the cell in SmAP_A phase shows Schlieren texture of both s=±l /2 and s=±l declinations [Fig. 1 1 inset (a)].

For the material under investigation, we obtained one of the display modes in SmAPR phase of the achiral bent-core liquid crystals. The SmAPR phase is macroscopically uniaxial, since the polar directors in the plane of the smetric layers are randomly ordered in the absence of the electric field. An application of rather higher electric field aligns the minor directors of this initially disordered structure and therefore induces biaxiality. This display mode is advantageous over the mode in biaxial nematic phase due to the absence of parasitic switching of the primary director since it has been anchored in the SmAPR by the layer structure itself. But the disadvantage of this switching mode is that it requires rather high electric field, which is necessary to align the initially disordered structure. Hence in order to achieve a reasonable value of the field induced optical biaxiality, electric field of 10 V/μηι or higher must be applied. In homeotropic geometry this corresponds to the voltage of several hundreds volts [Table 1 ]. From this point of view the SmAP_A phase is more useful due to the existence of the spontaneous biaxiality and its interaction with the electric field.

Fig. 1 1 shows the dependence of biaxiality on electric field and the corresponding textures under the crossed polarizers in the SmAP_A phase of a homeotropic cell under the in-plane electric field applied at a frequency of 1 10 Hz. At low values of electric field (<1.6 V/μηι), the initial schlieren texture transforms to a uniform biaxial texture [marked as (1 ), inset (b)], which with an increase in field goes to an intermediate uniaxial texture [marked as 2, inset (c)], a further increase in the field, the system goes to a ferroelectric state [marked as 3, inset (d)] with the same value of biaxiality as in the SmAP_A phase. The absence of biaxiality in the intermediate state has been explained on the basis that the field induced state is a distorted antiferroelectric structure with the short molecular axes in the neighbouring layers being disposed at an angle of 45° to each other. This sequence of transition of the biaxiality is exploited in driving the display either between the Low E bright state (1 ) to an intermediate E dark state (2) (mode 2a) or between the states (2) to a high E bright state (3) (mode 2b). The former type of switching could be achieved at very low fields, but switching time is rather large compared to the latter type of switching where we need to apply relatively moderate fields for faster switching. The electro- optic parameters of these modes are listed in Table 1.

Table 1

Possible Operating Driving Response Contrast Optimum δη display phase voltage time ratio cell

modes thickness

4 V/um (near 2 ms

1 SmAP_R SmAP_A > 1000: 1 100 μηι 0.003 at transition) 4ν/μηι

2 (a) 0.5-105 V/μιη ~ 8 ms 200: 1 16 μπι 0.016

SmAP_A

2(b) > 1.7 V/μιη ~3 ms 200: 1 16 μιη 0.016

3(a) < 1.5 V/μιη 0.5 ms > 1000: 1 16 μτη 0.016

SmAP_A

3(b) > 1.7 V/μηι < 0.4 ms > 1000: 1 16 μηι 0.016 Table 1. The possible LCD modes in SmAP_R and SmAP_A phases and their display parameters for C 64.

We have demonstrated different display modes in non-tilted, polar, antiferroelectric smectic phases of an achiral bent core compound with a lower threshold, high contrast, and continous grey scale. There is a considerable advantage to realising the switching in non-tilted biaxial smectic phases since the major director is already oriented by the smectic layers unlike in biaxial nematics where it needs to be anchored independently of the secondary director by external forces which may lead to parasitic effects like induced tilt with field. It is also possible to have higher switching speed in smectics, which is suitable for sequential colour display modes with LED backlight as electric field interacts with induced polarization as opposed to the dielectric biaxiality in nematics. Also since the phase is orthogonal in nature the problems associated with SmC materials (chevron structure, defects) can be avoided. The temperature at which the electroptical effect exists can be adjusted by modifying the chemical structure of the SmAP_A or SmAPp phases and/or by providing mixtures of different compounds. Ideally, the lower operating temperature should be below 0°C.

The entire contents of all of the documents listed in the Appendix are herein incorporated by reference in their entirety.

Modification and additions can be made to the embodiments of the invention described herein without departing from the scope of the invention. For example, while the embodiments described herein refer to particular features, the invention includes embodiments having different combinations of features. The invention also includes embodiments that do not include all of the specific features described.

The invention is not limited to the embodiments hereinbefore described, which may be varied in construction and detail. Appendix - References

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Claims

A liquid crystal display device comprising:- first and second substrates placed between cross polarisers, between said substrates, homeotropically aligned orthogonal biaxial smectic liquid crystal molecules having long axes and short axes, and an electrode drive arranged to cause in-plane switching of the direction of the short axes the electrode drive comprising: a first pair of electrodes which are arranged to generate a first in-plane electric field which is oriented parallel relative to the transmission axis of one of the polarisers; and a second pair of electrodes which are arranged to generate a second in-plane electric field which is oriented so as to deviate from the transmission axis of one of the polarisers.

A liquid crystal display as claimed in claim 1 wherein the first pair of electrodes are arranged to generate a first in-plane electric field which is oriented parallel relative to the transmission axis of one of the polarisers, and the second pair of electrodes are arranged to generate a second in-plane electric field which is oriented so as to deviate from the transmission axis of one of the polarisers by about 45°.

A liquid crystal display device as claimed in claim 1 or 2 wherein the first pair of electrodes are on the first substrate and the second pair of electrodes are on the second substrate.

A liquid crystal display device as claimed in claim 1 or 2 wherein the first pair of electrodes and the second pair of electrodes are arranged on a single substrate.

5. A liquid crystal display device as claimed in any one of claims 1 to 4 wherein the long axes of the liquid crystal molecules are perpendicular to the substrates.

6. A liquid crystal display device as claimed in any one of claims 1 to 5 wherein the short axes of the liquid crystal molecules are in the smectic layer plane which is parallel to the substrates in this geometry.

7. A liquid crystal display device as claimed in any one of claims 1 to 6 wherein the switching is about the long axes.

8. A liquid crystal display as claimed in any one of claims 1 to 7 wherein the liquid crystal molecules are homeotropically aligned anti ferroelectric smectic liquid crystal molecules.

9. A liquid crystal display substantially as hereinbefore described.