WO2013038150A1

WO2013038150A1 - Addressing arrangement

Info

Publication number: WO2013038150A1
Application number: PCT/GB2012/052186
Authority: WO
Inventors: Anthony Bernard Davey; William Alden Crossland; Andriy DYADYUSHA; Huan Xu; Daping Chu; Mykhaylo Pivnenko; Jonathan Paul Hannington; Terry Clapp
Original assignee: Cambridge Enterprise Limited; Dow Corning Corporation
Priority date: 2011-09-14
Filing date: 2012-09-06
Publication date: 2013-03-21
Also published as: CN204369798U; GB201115867D0; TW201317666A

Abstract

An optical device having a matrix arrangement of pixels defined by row electrodes and column electrodes, the row and column electrodes sandwiching a defined Sm A liquid crystal composition, the device further comprising drive circuitry connected to drive the row and column electrodes with alternating drive voltages, and addressing circuitry arranged to select pixels uses a one- third addressing scheme.

Description

Addressing arrangement

The present invention is in the field of photonics. An embodiment relates to an optical device using a smectic-A liquid crystal composition. In one non-limiting embodiment the optical device is a display. In another, it is an amplitude spatial light modulator.

The optical device contains a liquid crystal composition in which a disordered state is produced by the process of smectic-A dynamic scattering and a clear, uniform state is induced by dielectric re-orientation. Such optical devices can be used to provide variable amounts of light transmission- either locally, for example in "pixels" or across the whole device, and without the need for optical polarisers, Liquid crystals have molecules which tend to self order without freezing and thus gain crystalline attributes even though they still flow and may fill a container. The phases of liquid crystals are broadly a generalised sequence of states that such a molecular fluid may pass through on the way from being an isotropic liquid until it freezes as a solid, in general such molecules will be typified by strong anisotropy. The form that this anisotropy takes can he considered where the molecule is typified by a high aspect ratio (much longer than wide, thus "rod" or 1auY* like), and may have dipole character, and aiiisotropic polarisabUity. In these cases the average direction of molecular orientation is referred to as the "director". Nematic liquid crystals typify the commonest liquid crystalline materials and are commonly used in liquid crystal flat screen devices and flat-panel displays. Extending the length of nematic mesogens, or other structural changes, very often causes them to show further phases upon cooling below the nematic phase, and before solidification, and at lower temperatures the typical character may be of a "layered fluid". Such layered liquid crystals are called smectie" liquid crystals. Herein we will only consider the materials normally referred to as "smectic-A", abbreviated to "SmA", liquid crystals. For example the prototypical "5CB" (4^*- penfyl-4-biphenyl.carbonitrile), "5QCB" (is the ether linked pentyl, 4^>-(pentyloxy)- 4-b.ip enyi.carboriitrile). is nematic, the "SC8" (4^!~octyM-biphenylcarbonitrile) and "80CB" (4^,-(octyloxy)~4-hiphenylcarboaitrik), each exhibit a SmA phase beneath the higher temperature nematic phase, where in the abbreviation "mCB" and "mOCB":- m stands for an integer and refers to the number of carbon atoms in the alkyl or alkoxyi chain in 4-cyano-4^''-n-alkyIbiphenyl and 4-cyano-4'-n- alkoxybiphenyl, respectively; for example:

8CB - 4-cyano- *-octylbip.he»yl; and

SOCB- 4-cyano-4 '-octyloxybiphenyl

The molecules forming SmA phases have similar properties to those forming nematic phases. They are rod-like and usually have a positive dielectric anisotropy. The introduction of a strong transverse dipo!e in order to induce a negative dielectric anisotropy tends to destabilise the SmA phase and may lead to increased chemical instability.

Smectic-A liquid crystais exhibit hysteresis in their switching so that dielectric reorientation (or other disturbances of the smectic structure) does not relax when an applied electric field is removed. Unlike most nematic liquid crystal structures, dielectrically re-oriented SmA liquid crystals rest in the driven state until further forces are applied.

A panel may be formed by taking planar sheets, for example of glass, and applying to these a transparent conducting layer, typically made of indium tin oxide, the conducting layers being connected to conductors so mat a variable field may he applied. These two sheets may he formed into a panel for example separated by beads of uniform diameter (typically, say. 5-15 pre, dependent on desired cell thickness). This panel is then edge sealed with glue allowing one or more apertures for .filling with the liquid crystal material Using such a cell a mA liquid crystal layer may be formed by filling the panel (typically at an elevated temperature above the isotropic transition for the material). In the SmA devices discussed here, no alignment layers are required unlike nematic devices where uniform alignment of the cell is essential On fi lling and thermally cycling such a SmA panel from room temperature to above the isotropic transition and back again, the liquid crystal will exhibit textures that are typical for the phases. Whilst the nematic, with no surface alignment, may appear in the well-known Schlieren texture where line defects or 'threads' scatter light, in the SmA a 'focal coni c' texture is formed as a consequence of the layered structure of the SmA material. There is a sharp spatial variation in the refractive index which results in light scattering. The appearance of these textures results from the anisoiropy of the refractive index, which is highest when light is travelling orthogonal to the more poiarisable axis of the average molecular direction. The variation in refractive index causes light scattering. When viewed (under a microscope) between crossed poknsers, contrast can also be observed between regions of different molecular orientations.

To electrically address a SmA liquid crystal panel an alternating (AC) field is normally applied. In non-doped materials, positive dielectric anisoiropy of the LC will cause tiie re-arrangement of initially randomly aligned poly-domains, to align the mesogen with the field direction, (normal to the glass surface). The panel will appear clear, as the average orientation of the anisotropic molecules is normal to the glass layer. For most non-doped SmA materials this situation is only reversible by heating the cell to destroy the SmA alignment.

If a suitable ionic dopant is dissolved in the SmA liquid crystal host, then under the influence of DC or low frequency (e.g. 200 Hz) electric fields, two orthogonal forces attempt to orient the smectic A director:- i) Dielectric re-orientation as described above attempts to align the SmA director (indicating the average direction of the long .molecular axis) in the field direction. ii) Simultaneously, the movement of ions through the SmA electrolyte attempts to align the smectic A director in the direction in which ions find it easer to travel. in SmA materials this is within the layers, orthogonal to the field direction (i.e. the materials have positive dielectric anisotrqpy and negative conductivity anisotropy). The two competing forces give rise to an electro-hydrodyriamic instability in the liquid crystal fluid that is referred to as 'dynamic scattering'. In SmA materials the dynamic scattering state strongly scatters light and (in contrast to the similar state in nematic materials) the disruption of the SmA structure that it produces remains after the electrical pulse causing it has terminated. The reversibility between the clear, uniformly oriented, state and the ion-transit induced, poly-domain, scattering state, depends upon the different frequency domains in which these processes operate. Dynamic scattering requires the field driven passage of ions through the liquid crystal fluid. It therefore occurs only with DC or low frequency AC drive.

Higher frequencies cause dielectric re-orientation (the ions cannot "move*' at these frequencies) thus re-establishing a uniform orientation of the molecules. Thus the combination of dielectric re-orientation (into a clear transparent state) and dynamic scattering (into a strongly light scattering state) in a suitably doped SmA phase (possessing positive dielectric anisotropy and negative conductivit anisotropy) can form the basis of an electrically addressed display. High frequencies (typically >1O00 Hz) drive the SmA layer into an optically clear state and low frequencies (typically < 200 .Hz) drive it into the light scattering state. A key feature of such a display is that both these optical states are set up using short electrical addressing periods, and both persist indefinitely, or until they are re- addressed electrically. This is not true of the related phenomena hi nematic liquid crystals, it is this property of electro-optic bistabilHy (or more accurately rnulti- stability) that allows SmA dynamic scattering displays to be matrix addressed without pixel circuitry and which results in their extremely low power consumption in page-oriented displays or in smart windows,

The so called "one third select^*' mode will now be described in the context of a liquid crystal device having matrix of cells defined by the intersections of plural x electrodes with plural y electrodes. In ie 'one third selee mode of addressing all the y electrodes are held at a first voltage level (e.g. ground) except while they are being strobed in turn with, a second voltage level. The difference between the first and second voltage levels is the strobing voltage designated V_s. The x

electrodes are used for the input of data and are individual ly and selectively switched between third and fourth voltage levels respectively at ·+ Υ_ά and - V^. This arrangement of voltage levels is such that, when a row electrode \¾ is being strobed, the voltage ap earing across the elemental volume of liquid crystal x_a y_b> defined by the region where the column electrode ¾ crosses row electrode >_¾ is either (V_s - V_<j) ** V_e, a voltage sufficient to affect the liquid crystal material of that volume, or the smaller voltage (V_s -V^) depending upon whether column electrode x_a is held at the third or the fourth level during the strobing pulse. Irs the one third select mode of addressing V_s is made equal to twice ¼. The smaller voltage (V_s - V_<j) is insufficient to affect the volume.

Under these circumstances it is seen that the magnitude of the voltage applied across the elemental volume is eithe 3 = V_e or Y_d during a strobing pulse, and is equal to at other times. Thus the input of data is determined by the potentials appearing on the column electrodes while the row electrodes are being strobed. When any ro w e l ectrode other than y_b is being strobed, the magni tude of the voltage appearing across the elemental volume x_a Ή is only and hence it can only be switched during the strobing interval associated with its own row. la one aspect, there is provided a liquid crystal device having a matrix

arrangement of pixels defined by row electrodes and column electrodes, the row and column electrodes sandwiching a liquid crystal composition, the device further comprising drive circuitry connected to drive the row and column electrodes with alternating drive voltages, and addressing circuitry arranged to select pixels; the addressing circuitry being arranged to operate using so-called "one third select'^" or one-third addressing" technique in which it is arranged to apply a first alternating voltage to a row or respectively column to ^'be selected, a second alternating voltage to a column or respectively row to be selected and a third alternating voltage to a column or respectively row not to be selected, wherein the first alternating voltage has an amplitude substantially double that of the second and third alternating voltages, the first and second alternating voltages are mutually substantially in phase opposition, and the first and third alternating voltages are mutually in phase; the liquid crystal composition comprising, in weight %:

(a) 25 - 75% in total of at least one siioxane of the general formula i:

wherein

p ~ 1 to 10, e.g. 1 to 3,

q - I to 12, e.g. 6 to 10,

t - 0 or 1,

k - 2 or 3,

A is a phenyl or cyclohexyi ring which may be the same or different and are bonded together in para positions,

R a Cj.3 alkyl group, e.g. methyl, which may be the same or different,

X ^~ a C_W 2 alkyl group, and

Z - F, CI, Br, I CM. ¾, N0₂, MM©₂, NCS, CH₃, or OCH₃, CF_¾ <X¾ CW, Cl¾ especially CN; 0.001 - 1% in total of at least one quaternary ammonium salt of the general formula IT:

wherein:

T= a methyl group or a siiyi or siloxane group and

v— 1 to 30, for example v~ 9 to 19, e.g. m risi l (v-13, T~methyl) or cetyl (v=15 and T^raethyi),

Rl, R2 and R3, which may be the same or different, are

alkyl, e,g. methyl or ethyl,

Q is ail oxidative!}' stable ion, especially a OO₄ ion,

20-65% in total of at least one poiarisable linear molecule having an alkyl chain, the molecule having the general formula HI:

D--AV-Y (ΙΠ)

wherein:

D stands for a CM_$ straight chained alkyl or alkoxy group optionally containing one or more double bonds;

k ~ 2 or 3,

A* is a phenyl, cyclohexyl, pyrimidine, 1,3-dioxatie, or 1,4- bicycIo[2,2,2]octyi ring, wherein each A may be the same or different and are bonded together in para positions, the terminal ring attached to Y optionally being a phenyl and Y is located in the para position of the terminal ring of the group A^! _k and is selected from Z (as defined above in connection with Formula I), C- u straight chained alkyl, Cng straight chained alkoxy, OCHFj, NMe₂, Cl¾, OCOCH₃, and COCH₃; and 2 - 20%. optionally 5 - 15» in total of at least one side chain liquid crystal po!ysiloxane of the general formula IV:

I

(0)t— |Al_k -- Z

(IV)

wherein:

a. h and c each independently have a value of 0 to 100 and are such that a+b+c has an average value in the range 3 to 200, e.g. 5 to 20; and a is such that the chain units of the formula Y-R.₂SiO- SiR₂-O j_a represents 0 t 25 mole percentage of the compound of the general formula IV, and c is such that the units of the formula chain -{SiHR- OJ_e-RjSiO-Y represents 0 to 15 mole percentage of the compound of the general formula IV,

m - 3 to 20, e.g. 4 to 12;

t ^« 0 or 1,

k = 2 or 3

A is a phenyl or cydohexyl ring which may be the same or different and the rings are bonded together in para positions,

R ^~- a Cj-3 alky! group, e.g. methyl, each of which may be the sam or different, and

Y ^ a CM2 alkyi group, a chroniophore or a calamttic liquid crystal group and each of which may be the same or different, and

Z is as defined above in connection with Formula I. and wherein the amounts and nature of the components are selected such that the composition possesses smeetie A layering, as detected by X-ray diffraction. Such a composition has a relatively high and well-defined switching threshold, la other words the voltage gradient between electrodes of a cell containing the composition must reach a well-defined level before the composition is affected. Such a competition is well suited to the one-third select technique described herein, since the switching threshold (sometimes referred to as the "error voltage*') is high enough to permit it, as will he described later herein. However the

compositions will also work with other voltage regimes than "one-third select". The compositions described have been shown to retain the switching threshold over time and to continue to be able to operate after millions of switching events. The siloxane oligomeric moiety (a) may be a compound of the formula la:

(la) where X, R, p. and t are defined above in connection with Fonnul I and g and h each independently stand for 0, 1 or 2 and j stands for 1, 2 or 3, subject to the requirement that g+ +j is 2 or 3.

The side chain siloxane liquid crystal, component (d), which may be » polymer, copolymer or terpolymer. may be a compound of the general formula i Va

(IVa) where a, b, c, m and t are as defined hi connection with Formula IV, g ~ 0_» 1 or 2, h⁵⁵ 0, 1 or 2, j ~ 1, 2, or 3, subject to the requirement that g+h+j is 2 or 3; each R. may be the same or different and is an a!fcyl group, e.g. methyl; and Y - a Cj.g alky] group, a chromophore or a calamitic liquid cr stal group.

The ionic anion (b) of formula 0 may be a compound of the formula (Ha):

where v. Rl, R2, R3 and Q are as defined in claim 1 in connection with Formula

The ionic anion of fonnula Π may be a compound of the formula Ufa:

wherein, v, Rl. R2, R3 and Q are as defined in claim 1 or claim 4 in connection with Fonnula II and is a silyl or siloxane group. Component (c) may comprise an. organic ealamitic mesogen which exhibits cither a nemat c or a Smectio A liquid crystal phase.

The at least one polarisable linear molecule, component (c), ma include a compound of the formula Ilia and or a compound of the formula lllb.

where a ~ I to 15 and b ~ 1 to 13 ; - 0 or l,j ^::T 1 ,2 or 3 ; g ^« 0, 1 ,or 2, h ^ 0, 1.or 2 subject to the requirement that g+tri-j does not exceed 3.

The composition may further include:

(e) up to 10% by weight in total of at least one positive or negative dichroic dye, optionally a cyan, yellow, magenta, red, green or blue dye or an emissive dye, e.g. a fluorescent or phosphorescent dye, the dye being aligned with neighbouring mesogemc components of the composi tion.

The composition m include:

(f) up to 10% of one or more viscosity-reducing solvents or diluents.

The compositions may further include

(g) up to 10 wt% of at least, one molecule e.g. a lath-shaped molecule, that is not a liquid crystal, but which can be incorporated into the formulation, without degrading the smectic A l ayer quality of the composition . The at least one molecule that is not a liquid crystal may comprise a compound of the formula V):

The composition may also include:

(h) up to 50% by -weight, e.g. up to 40%, in total of at least one birefringence- alterin additive, e.g., birefringence- increasing additives, for example:

where R ^β C m alkyl, n ~ 0 or 1» L is selected from hydrogen, or Cj.3 alky! and X - C F, NCS, CF₃, OCF₃ or C,.₆ alkyl or

where R is a Cj„_$ i_¾ alkyl group,. or birefringenee-loweriag additives, for example:

alkyl group.

or

a Cum -alkyl group

where R^;;; a CMQ alkyi group The- total amount of the birefringence-altering additive component h) and the total amount of component (c) may be in the range of 35 - 73 t%, e.g. 40 - 65 wt% or 45 -60 wt%.

The composition may have a birefringence in the range 0.15 to 0.3, and preferabiy 0.16 to 0.2, at 20°C and 589rtm and be opaque in the disordered state and clear in the ordered state.

The composition may include up to 10% by weight in total of at least one positive or negative dichroic dye, optionally a cyan, yellow, magenta, red. green or blue or a black dye, or an emissive dye, e.g. a fluorescent or phosphorescent dye, the dye being aligned with neighbouring mesoge ic components of the composition.

The composition may have a birefringence in the range 0,07 to 0.15, and preferabiy 0.1 to 0.13, at 20°C and 589nm, (ii) is translucent in the disordered state and clear in the ordered state and ( i) includes up to 10% by weight in total of at least one positive or negative dichroic dye. optionally cyan, yellow, magenta, red, green or blue dye, or a black dye or an emissive dye, e.g. a fluorescent or phosphorescent dye, the dye being aligned with neighbouring mesogenic components of the composition.

in the drawings:

Figure I is a schematic view of an embodiment of liquid crystal device Figure 2 is a cross-section along the line Π-ΪΓ of Figure 1.

Figure 3 shows a block diagram of a drive and addressing arrangement for the device of Figure 1

Figure 4a-f show various waveforms provided by the arrangement of Figure 3.

in the figures like reference numerals refer to like parts. Referring to Figures 1 and 2, an embodiment of a liquid crystal device 100 lias two mutually spaced opposed substrates 210,220 sandwiching a liquid crystal material 200. in this embodiment both of the substrates are transparent to visible l ight and are of glass, thus being generally rigid, in other embodiments, transparency and rigidity may not be required or may be undesirable, and some embodiments use substrates of relatively flexible material, e.g. a polymer such as PET. The substrates 210 and 220 are secured together by a perimeter seal 230 and

maintained spaced apart by spacers (not shown).

As previous discussed, the liquid crystal material 200 is a thermotropie liquid crystal smeet!c A composition exhibiting a smeetic type A phase made up of multiple layers, wherein under the influence of different electric fields applied between the electrodes, the ali gnment of the layers of the composition can become more ordered or more disordered, the composition has stable states in which the alignment of the layers of the composition are differently ordered including an ordered state, a disordered state and intermediate states, the composition being such that, once switched to a given state by an electric field, it remains substantially in that state when the field is removed.

The smectie A liquid crystal composition may be a composition as described in PCT/US i 0/273 8, claiming priority from US patent application 61/314039, incorporated herein by reference, The liqui d crystal device 100 has plural mutually parallel column electrodes 1120 and plural mutually parallel ro electrodes 1.20 disposed generally perpendicular to the column electrodes 1 10. The electrodes are respectively disposed on the inwardly facing surfaces 205 and 215 of the substrates 210 and 220. The

intersection between a column electrode 1 10 and a row electrode 120 defines a pixel of the liquid crystal device 100. In this embodiment the device is dimensioned such thai a total voltage across a pixel of 100 volts is insufficient to affect the liquid crystal material (e.g. to cause a state change). The thickness of the liquid crystal material is typically in the range of 2 - 50 microns, e.g. 5-15 microns. The present embodiment is transmissive to visible light and hence the electrodes

110 and 120 are transparent, being for example of indium tin oxide (1TO).

In another embodiment the liquid crystal device 100 is a reflective device, having a substrate not transmissive to visible light and supporting reflective electrodes. A suitable material for such a substrate is silicon.

In this embodiment the set of mutually parallel column electrodes 110 and row electrodes 120 are strip electrodes that extend across the whole width/length of the substrates. Selection of a specific pixel can be performed by enabling the row electrode on which the desired pixel lies and enabling the column electrode on which it lies.

In practice plural pixels are usually selected at once, for example on a single row electrode. A low frequency voltage f_L (¾ <200 Hz, typically 50 Hz or 60 Hz mains frequency is used) serves to disorder the liquid crystal composition. A relatively high frequency f_¾ (¾> 1 kli¾ typically 2-4 kHz:) is applied to between the row and. column electrodes to cause the composition to become ordered.

In one method of operating the device 100, a circuit (not shown) provides the entire device with a mains frequency voltage to disorde all of the pixels of the device. This is achieved in an embodimetit by transforming the mains supply to a voltage of about 150 volts and applyin the resultant sinusoidal voltage between commoned row electrodes and commoned column electrodes. As explained later, sinusoidal voltage is not essential, but in the light of ac mains waveforms is convenient.

After disordering the device, selected pixels are then cleared to achieve the desired display.

F igure 3 is a block diagram o f an embodiment of addressing circuitry 300 for the liquid crystal device 100 having, in this embodiment, a logic controller 330, a column waveform generator 310 and a .row waveform generator 320.

Logic controller 330 has an input 33 L a first output 332 to the column waveform generator 310 and a second output 333 to the row waveform generator 320. The input 331 is used to cause the logic controller 330 to select a desired set of pixels along a predetermined row.

In this embodiment a single row electrodes 1 10a, containing pixels to be cleared is enabled. To do this, the selected row electrode 1 10a is supplied with a row selection waveform (Figure 4C) being a square wave having a frequency of 2 kHz with a amplitude excursion from HOC) volts to -100 volts.

All other row electrodes 120 are grounded (Figure 40), Column electrodes 110, for the selected pixels are driven with a pixel select waveform (Figure 4A) with an amplitude excursion from +50 volts to -50 volts, having the same frequency as the row select waveform, and in antiphase to it Column electrodes 1.10 for imselected pixels are supplied with a pixel imsekct waveform (Figure 4B) having an amplitude excursion from +50 volts to -50 volts, of the same frequency as the row select waveform, and in phase with it

Grounding the imselected rows ensures that charge cannot build u on imselected row electrodes- for example dm to crosstalk or residual charge remaining after selection has ceased; such charge could give rise to a sufficient potential difference to affect the state of unselected pixels.

Grounding unselected rows also means that all pixels in. those rows will be subject to a column voltage of either the pixel select waveform or the pixel unseleet waveform (in this case both having an amplitude excursion from +50 volts to - 50 volts), which is referred to in the art as the "error voltage". The particular compositions defined in this specification have a high error voltage tolerance- that is pixels will withstand a high error voltage without being affected by it. The ability to withstand a high error voltage means that the "'one-third select" regime can be used successfully. However other drive arrangements may be used with the composition, for example, those in which a lower or higher voltage is applied across unselected rows than a voltage of one third of the voltage used to clear the selected pixels.

The abi lity of these particular compositions to have an error voltage withstand level that remains high throughout a long life in terms of switching events is also very important

It will be seen that at any one instant the selected pixels have the sum of the voltage of the row select waveform and the voltage of the column select waveform across them (Figure 4E). This will be a voltage with an amplitude excursion from +150 volts to * 150 vote at 2 KHz Equally at any one instant the imselected pixels hav the sum of the voltage of the row select waveform 321 and the voltag of the column unselect waveform 312 across them (Figure 4F). This will he a voltage with an amplitude excursion from +50 volts to -50 volts at 2 KHz. A voltage of 50 volts is msufHeient to cause any effect on the pixels that are not to he cleared, as ft is below the selection threshold of the Smectic A liquid crystal composition. On the other hand the voltage excursion on the selected pixels is above the threshold of the liquid crystal c omposition and accordingly selected pixels are cleared. in this embodiment, simisoidal waveforms are used for homeotropic and focai- conic control of the liquid crystal device 100, due to lower peak current

requirements compared to square waveforms, due, in part, to less rapid rise and fell times. h this embodiment a mains sinusoidal waveform is used to disorder die pixels, and a 2 kHz sinusoidal waveform is used to clear the pixel. DC balance can be maintained by starting and stopping the waveform at a zero crossing point.

Example :

A 25 mm square test cell had a capacitance of approximately 10 nF, and required a 300 volts peak to peak 50/60 Hz dc balanced sinusoidal waveform applied for approximately 1 second to achieve the scattered state. During this time, the material presented an approx 20 parallel leakage resistance. Full clearing required an approx 1 - 4 kHz dc balanced waveform applied for several milliseconds at the same peak-peak voltage. The time taken is dependent on both ceil thickness and frequency- for example higher celt thickness is likely to take longer to clear/scatter. As the clearing frequency rises for a particular cell, the necessary voltage may reduce or the clearing time may reduce. As the scatter frequency falls towards dc, the necessary voltage may reduce or the scattering time may fall.

Drive waveforms may be of different shapes, although square (with controlled edge times) or sine-waves are most easily produced. Dc balance prevents degradation of the liquid crystal material The abi lity to ground electrodes (rather than leave floating) immediately after application of the waveform Is also a requirement.

Drive waveforms may be square waves, as well as sinusoidal, triangle and other waveforms, and their combinations. The wavesh ape of the waveform appl ied to the columns and selected row(s) need not be identical as long as the RMS requirements above and below the threshold voltage are met, and, for long term reliability, the signal across the liquid crystal is long-terra dc balanced.

Symmetrical waveforms are easier to generate, and dc balancing can he maintained by starting and stopping the

waveform at a zero-crossing point and including an integral number of cycles. Sine waves do not have the same peak current requirements as the rapid rise and fail times of square waves, bat need higher peak voltages to provide the same mis voltage.

Undetected rows are connected to waveform 0 volts (zero, ground), to prevent eapacitively-eoupled voltages exceeding the threshold voltage or disturbing dc balance. When no waveform is applied it is good practice to ground all rows and columns to improve lifetime. Waveform generators, either for pixeilated displays or unpixeliated large panels, are able to drive an alternating positive and negative voltage across the liquid crystal cell, and also clamp both sides to the same voltage, usually hut not necessarily ground (0 volts). An optical device may comprises a stack composed of two or more liquid crystal devices as previously described, stacked on top of each oilier, in an example, the ihermotropic liquid crystal smectie A composition in each device (i) contains up to 10% by weight in total of at least one positive or negative dichroic dye, optionally a cyan, yellow, magenta, red, green or blue dye, or a black dye or an emissive dye, e.g. a fluorescent or phosphorescent dye, the dye being aligned with neighbouring mesogenic components of the composition and (ii) is selected to exhibit a low birefringence in the range of 0.08 to 0.15, e.g. 0.1 to 0.13, at 20°C and 589nm.

Claims

CLAIMS i An optical device having a matrix arrangement of pixels defined by row electrodes and column electrodes, the row and column electrodes sandwiching a liquid crystal composition, the device further comprising drive circuitry connected to drive the row and column electrodes with alternating drive voltages, and addressing circuitry arranged to select pixels;

the addressing circuitry being arranged to operate using so-called "one third addressing" in which it is arranged to apply a first alternating voltage to a i t) row or respectively column to be selected, a second alternating voltage to a

column or respectiveiy row to be selected and a third alternating voltage to a column or respectively row not to be selected, wherein the first aitemating voltage has an amplitude substantially double that of the second and third alternating voltages, tire first and second alternating voltages are mutually substantially in

I S phase opposition, and the first and third alternating voltages are mutually in phase;

the liquid crystal composition comprising, in weight %:

(a) 25 formula I:

20

p ^~ I to 10, e.g. I to 3₅

q = i to 1.2, e.g. 6 to 10,

t *= 0 or 1,

k -™ 2 or 3 ,

A is a phenyl or cyclohexyl ring which may be the same or different, and are bonded together in para positions*

R~ a CM alky! group, e.g. methyl, which may be the same or different.

X ^s* a C|„i₂ alkyl group, and Z - F, CI, Br₅ 1, CN. N¾₅ N0₂, NMe₃, NCS, CH₃. or OCR CF₃, OCF_3? CH₂F_; CH¾ especially CN:

( b) 0.001 - 1% in total of at least one quaternary ammonium salt of the general formula II:

wherein:

T- a methyl group or a silyl or siloxane group and

v = 1 to 30, for example v= 9 to 19, e.g. tnyrisiyl (ν^«13, T^~met!iyl) or eety! (v=15 and T~methyl),

RL R2 and 3, which may be the same or different, are

alkyl e.g. methyl or ethyl,

Q is an oxidatively stable ion, especiall a CIO4 ion,

20-65% in total of at least one polarisable linear molecule having an alkyl chain, the molecule having the general formula HI:

D— A —Y (Hi)

wherein:

D stands for C_w<5 straight chained alkyl or alkoxy group optionally containing one or more double bonds; k ·^:· 2 or 3,

A ^' is a phenyl, cyclohexyl, pyrimidine. 1 ,3-dioxane, or 1 ,4- bteyclo[2,2,2]oetyJ ring, wherein each A may be the same or different and are bonded together in para positions, the temiinal ring attached to Y optionally being a phenyl and Y is located in the para position of the terminal ring of the group A and is selected from Z (as defined above in cormection with Formula I), C|._Jg straight chained alkyl, C_.M$ straight chained a!kox , OCHF_2s NM¾ CH₃, OCOCH_3s and €0C¾; and

2 - 20%, optionally 5 - 15, in total of at least one side chain liquid crystal poiysiloxane of the general formula IV:

(IV)

wherein:

a, b and c each, mdependenily have a value of 0 to 100 and are such that a-*-b-!-c has an average value in the range 3 to 200, e.g. 5 to 20: and a is such that the chain units of the formula Y«R₂SiO«[SiR2-0}₈ represents 0 to 25 mole percentage of the compound of the general formula IV, and c is such that the units of the formula chain -[SiH - 0|_c-R₂SiO-Y represents 0 to 15 mole percentage of the compound of t e general formula IV,

«1 = 3 to 20, e.g. 4 to 12;

t - 0 or 1,

k - 2 or 3

A is a phenyl or cyciohexyi ring which may be the same or different and the rings are bonded together in. para positions,

R ^ a Ct-3 alkyl group, e.g. methyl, each of which may be the same or different and

Y - a Cj-12 aiky! group, a chromophore or a calamitic liquid crystal group and each of which may be the same or different, and

Z is as defined above in connection with Formula Ϊ. and wherein the amounts and nature of the components are selected such tha the composition possesses smectic A layering, as detected by X-ray diffraction,

A device as claimed in claim ! , wherein the siloxane oligomeric moiety (a) a compound of the formul la:

(la) where X, R_» p, q and t. are defined above in connection with Formula I and g and h each independently stand for 0_? 1 or 2 and j stands for 1, 2 or 3, subject to the requirement thai g+h+j is 2 or 3.

3. A device as claimed i claim 1 or claim 2 wherein the side chain siloxane liquid crystal, component (d), which may be a polymer . copolymer or terpolymer, is a compound of fee general formula fVa

(IVa) where a, b, c, m and t are as defined in connection with Formula I V _f g ~ 0, 1 or 2.

0, 1 or 2J = 1, 2, or 3, subject to the requirement that g-f-h-tj is 2 or 3; each may be the same or different and is an alkyi group, e.g. methyl; and Y = a C_t._g alky! group, a chromophore or a calamitic liquid crystal group.

4, A device as claimed hi any one of claims 1 to 3, wherein the ioni anion (b) of formula 0 is a compound of the formula (11a):

where v, Rl_> R2, R3 and Q are as defined in claim I in connection with Formula II.

5. A device as claimed in any one of claims I. to 4, wherein the ionic anion of formula Π i s a compound of die .formula lib:

wherein v, Rl , R2, R3 and Q are as defined in claim 1 or claim 4 in connection with .Formula Π and is a silyi or siloxane group.

6. A device as claimed in any one of claims 1 to 5, wherein component (c) comprises an organic calamitic mesogcn which exhibits either a nematic or a Smectic A liquid crystal phase,

7. A device as claimed in any one of claims 1 to 6, wherein the at least one polarisable linear molecule, component (c}_} includes a compound of the formula i lia and/or compound of the formula Jb.

where a - 1 to 15 and b ~ 1 to 13; f™ 0 or L j - 1,2 or 3; g ~ 0,1, or 2, ^~ 0,1„or 2 , subject to the requirement that g+h i^~j d s not exceed 3.

8 A device as claimed in any one of claims I to 7, wherein the composition includes:

(c) tip to 10% by weight in total of at least one positive or negative dkhroic d e, optionally a cyan, yellow, magenta, red, green or blue dye or an emissive dye, e.g. a fluorescent or phosphorescent dye, the dye being aligned with eighbouring mesogenie components of the composition.

9 A device as claimed in any one of claims 1 to 8, wherein the composition includes:

(£} up to 10% of one or more viscosity-reducing solvents or diluents.

10 A device as claimed in any one of claims 1 to , wherein the composition includes:

(g) u to 10 wt% of at least one molecule e.g, a lath-shaped molecule, that is not a liquid crystal, but which can be incorporated into the formulation, without degrading the smectic A layer quality of the composition,

11. A device as claimed in claim 10, wherein the at least one molecule that is not a liquid crystal comprises a compound of the formula (V):

12. A device as claimed in any one of claims 1 to 11 , which also includes: (h) up to 50% by weight, e.g. up to 40%, in total of at least one birefringence- alterin additive, e.g. birefringence increasing additives, for example:

where R ^~ C^ alkyl, n - 0 or Ϊ,

where R = Cj.io alkyl, n - 0 or L I, is selected from hydrogen, or C₁.3 alkyl and X - CN, F_; NCS, CFj, OCF_? or alkyl or

where R is alkyl group,. or birefrin ence lowering additives, for example:

where R^™ a _i¾ alkyl group.

or

a Cue, alkyl group

where a€ _W alkyl group

13, A device s claimed in claim 1.2, wherein the total amount of the bireMngence-altering additive component, (h) and the total amount of component (c) is in the range of 35 - 73 wt%, e.g. 40 - 65 wt% or 45 - 60 wt%.

14. A. device as claimed in any one of claims 1 to 13, which has a birefringence in the range 0.15 to 0.3_» and preferably 0.16 to 0.2, at 20°C and 589nm and is opaque in a disordered state and clear in an ordered state.

15. A device as claimed in claim 14 which includes up to 10% by weight in total of at least one positive or negative dichroic dye, optionally a cyan, yellow, magenta, red, green or blue or a black dve, or an emissive dve, e.e. a fluorescent or phosphorescent dye, the dye being aligned with neighbouring rnesogenie components of the composition.

16. A device as claimed in any one of claims i to 14, which (i) has a birefringence in the range 0.07 to 0.15, and preferably 0.1 to 0.13, at 20°€ and 589mm. (ii) is translucent in a disordered state and clear in an ordered state and (iii) includes up to 10% by weight in total of at least one positive or negative dichroic. dye, optionally a cyan, yellow, magenta, red, green or blue dye, or a black dye or an emissive dye, e.g. a fluorescent or phosphorescent dye, the dye being aligned with neighbouring rnesogenie components of the composition.

17. A device as claimed in any preceding claim, wherein the spacing between the electrodes is in the range of 2 - 50 microns, e.g. 5-15 microns.