GB2359142A

GB2359142A - A liquid crystal display device

Info

Publication number: GB2359142A
Application number: GB0002733A
Authority: GB
Inventors: Michael John Towler; Elizabeth Jane Acosta; Harry Garth Walton; Craig Tombling; Martin David Tillin
Original assignee: Sharp Corp
Current assignee: Sharp Corp
Priority date: 2000-02-08
Filing date: 2000-02-08
Publication date: 2001-08-15
Also published as: GB0024636D0; GB0002733D0; GB2359143A

Abstract

A liquid crystal display device includes a layer of a chiral liquid crystal material (3) disposed between first and second substrates (1,1'). An active region (10) of the liquid crystal layer has a thickness d<SB>A</SB>. The liquid crystal layer also contains a nucleation region (11) in which the thickness of the liquid crystal layer is d<SB>N</SB> rather than d<SB>A</SB>. As a result of the difference in thickness of the liquid crystal layer between the active region and the nucleation region, the liquid crystal state that is stable in the active region in the absence of an applied voltage is not the same as the stable state in the nucleation region. In one embodiment of the invention, the thickness of the liquid crystal layer is greater in the nucleation region (11) than in the active region (10). This embodiment can easily be realised in a reflective display device that is provided with a via-hole by using the via-hole as the nucleation region.

Description

2359142 1 A liquid crystal display device The present invention relates to

a liquid crystal display device, and in particular to a surface mode LCD such as a pi-cell device or a splay-bend device (SBD).

The term "surface mode LCW as used herein means a LCD in which the optical change caused by varying the electric field across the liquid crystal layer occurs primarily in the surface layers of the liquid crystal. Examples of surface mode LCDs are the pi-cell and the splay-bend device, although other types of surface mode LCDs are known. Surface mode WDs are disclosed in "Sov. J. Quantum Electronics", 1973, Vol 3, p78-79.

The pi-cell (otherwise known as an "optically compensated birefringent device" or OCB) is described in "Mol. Cryst. Liq. Cryst.", 1984, Vol 113, p329-339, and in US patent 4 635 05 1. The structure of a pi-cell is schematically illustrated in Figure 1. The device comprises transparent substrates 1, 1' on which are disposed alignment layers 2, 2'. A layer of nematic liquid crystal 3 is disposed between the substrates 1, 1'.

The alignment layers 2, 2' create parallel alignment of the liquid crystal molecules in the liquid crystal layer 3 at its boundaries with the alignment layers 2, 2'. This can be achieved by using parallel-rubbed polyamide alignment layers.

Addressing electrodes (not shown) are provided on the substrates 1, V, so that an electric field can be applied to selected areas of the liquid crystal layer. The liquid crystal layer 3 is placed between linear polarisers 4, 4', whose transmission axis are crossed with one another and are at 45' to the optic axis of the liquid crystal layer.

A retarder 5, with its optic axis parallel to the optic axis of the liquid crystal layer, may optionally be provided to compensate for the retardation of the liquid crystal layer. The retarder lowers the required range for the operating voltage by allowing zero retardation of the LCD to be achieved at a finite voltage across the liquid crystal layer.

2 Figure 1 shows a transmissive LCD. A pi-cell can also be embodied as a reflective device by providing a reflector below the liquid crystal layer, possibly by making the addressing electrode on the lower substrate a reflective electrode. The lower polariser 4' is not required with a reflective pi-cell.

The principle of operation of the pi-cell device is illustrated in Figures 2(a) to 2(c).

When no electric field is applied across the liquid crystal layer, the liquid crystal is in an H-state (homogenous state or splay state), in which the liquid crystal molecules in the centre of the liquid crystal layer are substantially parallel to the substrates. This is shown in Figure 2(a). The short lines in the figures represent the director of the liquid crystal molecules.

When an electric field greater than a threshold value is applied across the liquid crystal layer, the liquid crystal molecules adopt a V-state (or a bend state). In this state, the liquid crystal molecules in the centre of the liquid crystal layer are substantially perpendicular to the substrates. Figure 2(c) shows a first V-state which occurs at a low applied voltage across the liquid crystal layer, and Figure 2(d) shows a second V-state which occurs when a higher voltage is applied across the liquid crystal layer. The picell is operated by switching the liquid crystal layer between the first, low voltage Vstate and the second, higher voltage V-state.

If the electric field across the liquid crystal layer should be reduced below the threshold value, the liquid crystal layer will relax to the Hstate of Figure 2(a); in order to recommence operation of the device, it is necessary to put the liquid crystal layer back into the V-state. This generally requires a large applied voltage, owing to the low pretilt of the liquid crystal molecules. The pre-tilt is usually below 45' and typically between 2 and 10' so as to provide sufficient optical modulation and fast switching between the two V-states (for instance of the order of a millisecond or less).

One problem with known OCB devices is the difficulty of nucleating and stabilising the V-state, which is topologically distinct fTom the H-state. One prior art technique is

3 described in UK Patent Application 9521043.1/2 306 228. In this prior art technique, the V-state is nucleated under the application of a high voltage, and is stabilised by the polymerisation of a network whilst a high voltage is applied. This prior art technique is, however, unsuitable for use in active matrix devices, since it is difficult to apply voltages having the required magnitude in a TFT panel. A further disadvantage is that the in-situ polymerisation can lead to ionic contamination of the liquid crystal layer, and result in image sticking.

The SBD device, which is also a surface mode device, is described in UK Patent Application No. 9712378.0/2 326 245. The structure of a SBD device is generally similar to that of a pi-cell, except that the aligninent layers in a SBI) device have a high pre-tilt whereas the alignment layers in a pi cell have a low pre-tilt. An SBD device uses a liquid crystal material with a negative di-electric anisotropy, whereas a pi-cell uses a liquid crystal material having a positive di-electric anisotropy.

The principle of operation of a SBD is analogous to that of a pi-cell. When no voltage is applied across the liquid crystal layer of an SBD, the stable liquid crystal state is a Vstate. When an electric field greater than a threshold value is applied across the liquid crystal layer, an Hstate becomes stable. The SBD is operated by switching the liquid crystal between a first H-state which occurs at a low applied voltage across the liquid crystal layer and a second H-state which occurs when a higher voltage is applied across the liquid crystal layer. If the electric field across the liquid crystal layer is reduced below the threshold value the liquid crystal will relax into the V-state, and it will be necessary to put the liquid crystal back into the H-state before operation can be recommenced.

The high pre-tilt alignnient layers required for a SBD can be produced, for example, by the photo-polymerisation of a mixture of reactive mesogens.

SID 97 Digest, page 739, discloses a method of promoting nucleation of the V-state in a pi-cell. Voltages of the order of 20V are applied across the liquid crystal layer to switch 4 the liquid crystal from the H-state to the V-state. However, it is difficult to provide voltages of this magnitude in a TFT (thin film transistor) substrate.

Japanese published Patent Application JP-A-9 90432 (Toshiba) discloses the provision of nucleation sites within a pi-cell panel. The nucleation sites are provided by including spacer balls or pillars within the pi- cell panel, and cooling the liquid crystal material from an isotropic phase to a nematic phase while an electric field is applied across the panel. This results in some of the spacer balls/pillars acting as nucleation sites for growth of the V-state into the H-state. This prior art has a number of disadvantages. Firstly, it requires additional process steps during fabrication of the panel, since it is necessary to align the liquid crystal molecules under the influence of an applied electric field. These additional process steps complicate the fabrication of the panel. Secondly, some spacer balls/pillars can nucleate the H- state into the V-state thus destabilising the operating state of the panel.

Miwa et al disclose, in IDW 97-Digest page 85, a method of maintaining the stability of a V-state in a pi-cell. A resetting period is provided within each frame, and the high voltage V-state is addressed in this period. This prevents the liquid crystal layer relaxing to the H-state when low driving voltages are applied. This does not, however, address the initial nucleation of the V-state from the H-state.

US patent No 4 566 758 discloses a surface mode nematic liquid crystal display device in which the liquid crystal layer contains a chiral dopant. When no voltage is applied across the liquid layer the liquid crystal relaxes to a twist state rather than to an H-state. The problems associated with nucleating the V-state is reduced or even eliminated, since the twist state is topologically equivalent to the V-state. However, this approach requires that the ratio of the thickness d of the liquid crystal layer to the pitch p of the twist of the liquid crystal molecules must be d/p > 0.25, and such a high d/p value Is known to reduce the brightness of the display. It is not possible to reduce d/p below 0.25 since, in order to obtain a 180' twist, it is necessary to bias the pitch of the liquid crystal molecules enough to favour thetwist angle of 180' over a twist angle of 0% and this requires the value of d/p to be greater than 0.25. (It is necessary for the liquid crystal to conform to the rubbing directions of the alignment films on the two substrates, so that the liquid crystal layer in a pi-cell geometry is constrained to have a twist of 0% 180', 360% etc.) Co-pending UK Patent Application No. 9822762.2 discloses a surface mode liquid crystal display device in which the liquid crystal layer contains nucleation regions in which the desired operating state is stabilised. These nucleation regions are obtained by providing regions of different pre-tilt angle on at least one of the substrates. Such a device has the disadvantage that additional processing steps are required in order to provide the regions of different pre-tilt angle that produce the nucleation regions.

The present invention provides a liquid crystal display device compn sing: a liquid crystal layer disposed between first and second substrates, and means for applying a voltage across the liquid crystal layer; wherein a first region of the liquid crystal layer is an active region for display and a second region of the liquid crystal layer is a nucleation region for generating a desired liquid crystal state in the first region when a voltage is applied across the liquid crystal layer, the first region of the liquid crystal layer having a first thickness dA and the second region of the liquid crystal layer having a second thickness dN different from the first thickness.

The second region acts as a nucleation region. The stable state in the second region when no voltage is applied across the liquid crystal layer is chosen such that, when a voltage is applied across the liquid crystal layer, the desired stable state is generated in the active region at a lower applied voltage than in the absence of a nucleation region.

The second thickness may be selected such that, when no voltage is applied across the liquid crystal layer, the liquid crystal state stable in the second liquid crystal region is topologically equivalent to the desired liquid crystal state. This facilitates the process of nucleating the desired stable state in the active region.

The first and second thicknesses may be selected such that, when no voltage is applied across the liquid crystal layer, a first liquid crystal state having a first twist angle is 6 stable in the first liquid crystal region, and a second liquid crystal state having a second twist angle different from the first twist angle is stable in the second liquid crystal region.

The twist of the liquid crystal layer may be in the same sense as the natural twist of the liquid crystal material and the second thickness may be greater than the first thickness. The first liquid crystal state may be a 0' twist state and the second liquid crystal state may be a 180' twist state.

As noted above, the 180' twist state is topologically equivalent to the desired operating states of a pi-cell. When the invention is applied to a pi-cell, when a voltage is applied across the liquid crystal layer the desired operating state will grow from the 180' twist state contained in the nucleation region.

The alignment direction on the first substrate may be parallel to the alignment direction on the second substrate, and the first and second thicknesses may be selected such that dA/P < 0.25 and 0.25:5 dN/P:5 0.75, where p is the pitch of the liquid crystal material. These alignment directions will stabilise the 0' twist state in the first region and the 180' twist state in the nucleation region.

The ratio dA/P may be less than 0. 125, and it may be less than 0. 1. Use of a low d/p ratio means that the brightness of the active region will not be significantly reduced.

The 0' twist state may be an H-state.

The device may be a surface mode liquid crystal display device. The device may be a pi-cell. The desired liquid crystal state in the active region may be a V-state.

The device may be a reflective liquid crystal display device. The second liquid crystal layer region may be disposed at a via-hole. It is normal for a reflective liquid crystal display device to incorporate via holes, for example to enable electrical connection to a reflective electrode. The thickness of the liquid crystal layer in a via-hole region will be 7 greater than the thickness of the liquid crystal layer elsewhere, so that a via-hole region can be used as a nucleation region in a liquid crystal display device according to the present invention. In this embodiment, no additional processing steps are required to obtain a region of increased thickness of the liquid crystal layer to act as the nucleation region.

The twist of the liquid crystal layer in the second region of the liquid crystal layer may be non-zero and may be opposite to the natural twist of the liquid crystal molecules, and the second thickness may be smaller than the first thickness.

The alignment direction on the first substrate may be at an angle to the alignment direction on the second substrate, and the first and second thicknesses may be selected such that:

dA < 0 -1; and p 2.z 4 dN:> 0 - 1 p 2.z 4 where p is the pitch of the liquid crystal material.

Preferred embodiments of the present invention will now be described by way of illustrative example with reference to the accompanying figures in which:

Figure 1 is a schematic sectional view of an OCB device (pi-cell); Figures 2(a) to 2(d) illustrate the principle of operation of an OCB device; Figures 3(a) and 3(b) are schematic sectional views illustrating the principle of the present invention; 8 Figure 4 is a schematic sectional view of a liquid crystal display device according to an embodiment of the invention when no voltage is applied across the liquid crystal layer; Figure 5 is a schematic sectional view of the liquid crystal display device of Figure 4 when a voltage is applied across the liquid crystal layer; Figure 6 is a schematic plan view of a liquid crystal device according to an embodiment of the invention; Figure 7 shows the dependence of the speed of propagation of a dislocation in a liquid crystal material as a function of the d/p ratio; Figure 8 is a schematic sectional view of a liquid crystal display device according to a further embodiment of the present invention; Figure 9 is a schematic perspective view of the principal optical components of the liquid crystal display device of Figure 8; and Figure 10 shows the characteristic curve of reflectance against applied voltage for a reflective liquid crystal display, for various values of the d/p ratio.

Figure 2 illustrates the principle of operation of an OCB device. As noted above, the operating states are the two V-states shown in 2(c) and 2(d), and the device is operated by switching the liquid crystal state between these two V states. It is necessary to apply a finite voltage across the liquid crystal layer in order to obtain either of the V-states and if the applied voltage is reduced below a critical threshold value then the liquid crystal relaxes to the H-state shown in Figure 2(a). In order for operation of the device to be recommenced, the liquid crystal layer must be put back into a V-state.

When the voltage applied across the liquid crystal layer is reduced below the threshold voltage, a 180' twist state as shown in Figure 2(b) appears initially, before the appearance of the H-state shown in Figure 2(a). The 180' twist state is topologically 9 equivalent to the V-states shown in Figures 2(c) and 2(d), but is topologically distinct from the H-state of Figure 2(a). The 180' twist state is stable only for a small range of applied voltage, and as the voltage is reduced to zero, the 1801 twist state is superseded by the H- state.

The presence of the 1800 twist state during the transition from a V-state to the H-state has generally not been considered to be relevant to, or useful in, the operation of a picell. The present invention, however, aims to stabilise, at zero applied voltage, the 180' twist state within a specific region of a liquid crystal layer. When a voltage is applied across the liquid crystal layer this region will act as a nucleation region and generate the V-state which can then grow into an active region of the device (or into remainder of the active region if the nucleation region is disposed with the active region).

The principle of the invention is illustrated schematically in Figures 3(a) and 3(b), which show a liquid crystal display device according to the invention in the state of zero applied voltage and a non-zero applied voltage respectively.

The present invention uses a liquid crystal layer that contains a twisted liquid crystal material. This can be either a combination of a liquid crystal material such as a nematic liquid crystal material and a chiral dopant, or it can be a liquid crystal material that is intrinsically chiral such as a cholosteric liquid crystal material.

The liquid crystal layer does not have a constant thickness. The active region 10 has a thickness dA, and this thickness is selected such that the thickness-to-pitch ratio of the active region satisfies dA/p < 0.25. The liquid crystal state in the active region is therefore not a twist state, and will be an H-state as in a conventional pi-cell.

The nucleation region 11 has a thickness dN that is greater than the thickness of the active region. The thickness of the nucleation region is selected such that 0.25:5 dN/p:! 0.75. As a result, a 180' twist state is stabilised within the nucleation region when no voltage is applied across the liquid crystal layer. A disclination 12 exists at the boundary between the 180' twist state of the nucleation region 11 and the H-state of the active region 10.

In order to put the device of Figure 3 into its operating state, it is necessary to apply a voltage across the liquid crystal layer that is greater than a threshold value at which the Gibbs free energy of the Hstate and the V-state are equivalent. When a voltage equal to this threshold voltage is applied, the relative stability of the states changes to favour the V-state over the H-state. The V-state, which can be considered as a distorted twist state, can then grow from the 180' twist state in the nucleation region 11 into the active region 10, and will replace the H-state throughout the active region.

The threshold voltage required to nucleate the V-state into the active region 10 in a device of the type described in Figure 3 is typically around 2V. This is a considerably lower voltage than required to nucleate the V-state in a conventional pi-cell not provided with a nucleation region.

The thickness-to-pitch ratio of the active regions of the liquid crystal device shown in Figure 3 is below 0.25, and can typically be reduced to around 0. 1. The brightness of the active region of the device is therefore greater than an active region of a device of the type disclosed in US Patent No. 4 566 758 in which the d/p ratio must be at least 0.25 in order to stabilise the 180' twist state in the active region.

It is particularly easy to apply the present invention to a reflective liquid crystal display device, since these generally have a liquid crystal layer that contains a region of increased thickness as a result of a need to provide a via hole to allow a reflective electrode to be connected to, for example, a switching element. Using the via-hole as the nucleation region, eliminates the need for additional processing steps in the manufacture of the nucleation region.

Figure 4 is a cross-sectional view of a reflective liquid crystal display device embodying the present invention. This shows a via-hole region 11 disposed within an active region 10. The thickness of the liquid crystal layer in the active region is dA, and in the via- 11 hole region is dN. In this embodiment of the invention, the via-hole region is used as a nucleation region.

The device shown in Figure 4 comprises upper and lower substrates 1, P. An upper electrode 6 is disposed on the upper substrate, and this is covered by an upper alignment film 2. The electrode 6 is a transparent electrode, for example formed of indium tin oxide (ITO). The upper alignment layer 2 is a conventional alignment layer, and is for example formed of a layer of polymeric matenial that is unidirectionally rubbed so as to define the alignment direction and pre-tilt angle of liquid crystal molecules in contact with the upper alignment film 2.

A reflective electrode layer 7 is disposed on the lower substrate. A lower alignment layer 2' is disposed over the lower electrode. An electrode 6% for example the output electrode of a switching element (not shown) in the case of an active matrix display device is disposed on the lower substrate F. In the active regions of the device the reflective electrode 7 is separated from the electrode 6' by a layer 5 of an insulating material. In the via-hole region, however, the insulating layer 5 is not provided so that the reflective electrode 7 makes electrical contact with the electrode C. As a consequence of the absence of the insulating layer 5 in the via-hole region 11, the thickness of the liquid crystal layer in the via hole region is greater than the thickness of the liquid crystal layer in the active region 10.

In the embodiment of Figure 4 the rubbing direction of the upper alignment film 2 is parallel to the rubbing direction of the lower alignment film T. The liquid crystal layer is therefore constrained to adopt states having a twist angle of 0% 180% 360' etc.

In the embodiment of Figure 4 the liquid crystal layer contains a nematic liquid crystal and a chiral dopant. Although the nematic liquid crystal does not have an intrinsic twist, the presence of the chiral dopant induces a twist in the liquid crystal material. A suitable liquid crystal material is, for example, the material ZLI 6000 - 100, and a suitable chiral dopant is the dopant CB 15 (produced by Merck, Darmstadt, Germany).

12 The amount of chiral dopant in the liquid crystal layer is chosen such that the thicknessto-wpitch ratio in the active region 10 is less than 0. 25. This is not sufficient to bias the liquid crystal material into the 180' twist state, so the stable liquid crystal state is constrained to be a 0' twist state. Thus, at zero applied voltage the active region behaves like a conventional OCB device, and the stable state will be an H-state.

The pitch of the twist of the liquid crystal material is also selected so that the thicknessto-pitch ratio in the via - hole region 11 is greater than, or equal to 0.25 but less than 0.75, so that the liquid crystal material in this region is biased into a 180' twist state.

As noted above, the brightness of a liquid crystal layer decreases as the amount of chiral dopant is increased, so that it is preferable to minimise the amount of chiral dopant used, subject to the d/p ratio in the nucleation region being dN/p t 0.25 so as to stabilise the 180' twist state in the nucleation region.

Diselinations 12 are present at the boundary between the 180' twist state of the via-hole region and the H-state of the active region.

Figure 5 shows the liquid crystal display device of Figure 4 when a voltage is applied across the liquid crystal layer. When a voltage greater than the critical threshold voltage referred to above is applied, the V-state 13 grows from the via-hole region 11 into the active region 10. The disclinations 12 propagate away from the boundary between the viahole region and the active region, and eventually the V-state is the stable state throughout the active region 10. To nucleate the V-state into the active region 10 from the via-hole region 11 it is in principle sufficient for the applied voltage to just exceed the threshold voltage. However, the speed of growth of the nucleated V-state would be very slow at a voltage that only just exceeds the threshold voltage, and hence it is preferable to use a voltage significantly greater than the threshold voltage in order to reduce the time taken for the V-state to nucleate into the active region 10.

13 Once the V-state has nucleated into the active region, the device can be operated between a low voltage V-state and a high voltage V-state in the conventional manner described with reference to Figures 2(c) and 2(d).

The thicknesses of the active region 10 and the via-hole region 11, and the pitch of the twist of the liquid crystal material, can be selected to give any values that satisfy the requirements:

dA/P < 0.25 0.25: dN/p:! 0.75 (1) and (2).

However, as noted above, it is preferable for the d/p ratio in the active region to be low, in order to minimise the amount of chiral dopant and improve the brightness of the active region. As an example, the liquid crystal layer in the via-hole region 11 may have a thickness of 6gm, and the pitch of the liquid crystal molecules may be 24gin, so that the ratio dN/P = 0.25. If the thickness of the liquid crystal layer in the active region 10 of the pixel is 3 gm, then the d/p ratio in the active region is 0. 125.

For clarity, the invention has been described above with reference to a liquid crystal display device that incorporates one active region and one nucleation region. The invention is not limited to this simple case, however, and a practical display device will generally incorporate a large number of active regions each of which may be provided with a nucleation region in accordance with the invention.

Figure 6 is a plan view of a liquid crystal display device according to the present invention. It will be seen that the device contains a plurality of independently addressable active regions 10. The active regions can be produced by, for example, patterning the reflective electrode 7 so as to provide a plurality of independently addressable electrodes each defining an independently addressable region, or pixel, of the liquid crystal display device. The reflective electrode 7 in each active region 10 is provided with a via-hole 11 to enable electrical connection to a switching element.

14 Thus, the device of Figure 6 is a reflective, pixelated active matrix liquid crystal display device, having short nucleation times. The device can be used, for example, as a projection apparatus, or as a helmet mounted display device.

A further advantage of the present invention is that, if the voltage applied across the active region is reduced to a value such that the Hstate becomes the stable state, then the speed at which a randomly nucleated region of the H-state grows back across the active region will be slower than in a conventional pi-cell. This is shown in Figure 7, which illustrates the speed of propagation of a boundary of a growing H-state in a picell having a liquid crystal layer thickness of 6.2gin at a temperature of 25'C. Figure 7 relates to the liquid crystal material ZL16000-100 and shows the variation in the speed of the boundary as the d/p ratio of the liquid crystal material is altered by the addition of the chiral dopant CBI 5.

It will be seen from Figure 7 that increasing the d/p ratio decreases the speed of propagation of the boundary of the H-state. Thus, the speed of propagation of the boundary of an H-state region in the active region 10 of the device of Figure 4 with d/p > 0 will be slower than the speed of propagation in a conventional pi-cell having a d/p ratio of zero. A further embodiment of the invention will be described with reference to

Figures 8 and 9. This embodiment relates to a liquid crystal display device having a liquid crystal layer, containing a chiral dopant that induces a twist in the opposite direction to the twist induced by the rubbing directions of the alignment films. Such a device is disclosed in co-pending UK Patent Application No. 9828809.5 and Japanese Patent Application No. BH 11 3 71 963, filed on 27 December 1999, the contents of which are hereby incorporated by reference.

The device shown in Figure 8 is of the reflective single polariser type and may be used as, for example, a pixelated liquid crystal display. The device comprises upper and lower substrates 1, F. Thd upper substrate 1 carries on its inner surface an electrode 6 and an alignment layer 2, for example comprising a rubbed polyimide.

The lower substrate 1' carries on its inner surface a reflective electrode 6', although a separate electrode and reflector may alternatively be provided. An alignment layer 2% for example of the same type as the alignment layer 2 on the upper substrate 1, is formed on the reflective electrode 6. The upper and lower substrates 1, 1' and the associated layers are spaced apart, for example by spacer balls (not shown), to define a cell containing a liquid crystal layer 3. The layer 3 comprises a chiral liquid crystal material. This may be an inherently chiral liquid crystal material, or it may consist of a nematic liquid crystal to which has been added a chiral dopant.

As shown in Figure 9, the polariser 4 has a linear polarising direction 15. The upper alignment layer 2 has an alignment direction 16 which is oriented at a clockwise angle 0 with respect to the polarising direction 15 of the polariser 4. The lower alignment layer Thas an alignment direction 17 which is oriented at a clockwise angle of (0 +) so that the liquid crystal layer 3 has a twist of in a clockwise or positive direction.

The chiral dopant added to the nematic liquid crystal of the layer 3 is such as to have a twisting effect in the anti-clockwise or negative direction in the device shown in Figure 8. Thus, if the liquid crystal material of layer 3 were not constrained by the alignment layers 2, 2% it would adopt an anti-clockwise twist because of the chiral dopant. However, the aligmnent layers 2, 2' induce a positive or clockwise twist of less than 90' on the liquid crystal material in the layer 3.

If too much chiral dopant were added to the liquid crystal resulting in too small a value of d/p for the liquid crystal layer 3, then the twist energy of the material of the liquid crystal 3 would be larger than the twist energy of the twisted state induced by the alignment layers 2, 2' and the twist of the liquid crystal layer 3 would change from ' to ( n)', since > 0. The amount of chiral dopant which may be added depends on the twist angle and tends to zero as the twist angle tends to 90'. The critical d/p ratio (d/p), is given by:

d) < 0 _1 (3) p, - 2.z 4 16 First and second fixed retarders 20, 21 are disposed in the optical path between the polariser 4 and the liquid crystal layer 3. In Figure 8, the retarders 20, 21 are disposed between the substrate 1 and the electrode 6 so as not to affect the operating voltage of the device. However, it is in principle possible for either or both of the retarders 20, 21 to be disposed between the electrode 6 and the alignment layer 2, in which case the operating voltage of the device would be increased, or between the polariser 4 and the substrate 1, in which case the acceptance angle of the display would be reduced.

The retarder 20 has an optic axis 22 disposed at an angle a to the polarising direction 15 of the polariser 4, whereas the retarder 21 has an optic axis 23 at an angle P to the polarising direction 15.

Figure 10 shows the reflectance of a display device of the type shown in Figures 8 and 9. The results of Figure 10 were obtained with a display in which the first retarder 20 has its slow optic axis at 15' to the polarisation direction 15 of the polariser 4, and has a retardation A n.d = 260nin. The second retarder has its slow optic axis at 75' to the polarisation direction 15, and has a retardation A n.d = 95mn. The liquid crystal layer 3 has a twist angle = 70', and the angle 0 between the polarisation direction 15 and the rubbing direction of the upper alignment layer 2 is 0 = 40'. The retardation of the liquid crystal layer is A n.d = 20Onin. The thickness of the liquid crystal layer is 3gm.

As can be seen from Figure 10, the voltage required to obtain a particular reflectance below the reflectance minimum decreases as the d/p ratio of the liquid crystal layer is decreased from 0.

In the absence of pre-tilt induced in the liquid crystal layer by the alignment layers 2, 2% the dopant limit at which the twist of the liquid crystal layer will change from ' to is given by equation (3) above. If the alignment layers induce a non-zero pre-tilt in the liquid crystal molecules adjacent the alignment layers, the magnitude of (d/p)c is increased.

17 This embodiment relates to a liquid crystal display device in which the desired operating state has a twist angle where 0 < < 90', with being defined by the alignment directions of the alignment layers 2, 2'. As can been seen from Figure 10, adding a chiral dopant that induces a twist in the opposite direction to the twist induced by the alignment layers (that is, adding a chiral dopant having a pitch p that is negative) leads to a reduction in the reflectance voltage. However, it can be seen from Figure 10 that the greater reductions in the reflectance voltages are obtained at high dopant concentrations (thus giving a value for d/p that is much less than zero since p is negative). As explained above, a high level of dopant concentration will lead to the stabilisation of an undesired ( - n)' twist state at zero applied voltage. Therefore, before operating the device it is necessary to nucleate the ' twist state, and this nucleation is facilitated by providing a nucleation region in which the ' twist state is stable at zero applied voltage. Upon application of a voltage above a threshold value, the twist state will spread out from the nucleation region into the active region and displace the unwanted 7c)' twist state from the active region.

For = 70% the critical value of the thickness to pitch ratio is (d/p)c = 0.055. If the liquid crystal layer is reverse doped beyond this limit, the stable twist state in the presence of no applied voltage has a twist ( n)' = -110'. In order to access the desired operating state having twist in a device in which the liquid crystal layer is reverse doped beyond the critical value, the o twist state must first be nucleated and propagated into the active region where the ( - 7r)o twist state is stable at zero applied voltage, by application of a voltage greater than the threshold voltage to the liquid crystal layer.

According to the present invention, the device illustrated in Figures 8 and 9 is provided with a nucleation region for nucleating the twist state. The thickness of the liquid crystal layer in the nucleation region is different from the thickness of the liquid crystal layer in the active region. In this embodiment, however, the thickness of the liquid crystal layer in the nucleation region is less than the thickness of the liquid crystal layer in the active region.

18 The reduced thickness of the liquid crystal layer in the nucleation region may be achieved, for example, by adding small bumps or pillars of a photo-sensitive polymer to create nucleation regions having a lower thickness than the active region. The amount of doping would be selected so that the d/p value was the same as or lower (that is, more negative) than the critical value (which is negative) in the active region, but was greater than the critical value in the nucleation region.

For the device described above having dA=3im, the nucleation region could be provided by adding some surface relief having a height of 2PLm on one of the alignment films. This would produce a region in the liquid crystal layer having a thickness of only 1 gm. The amount of doping would be chosen to give a d/p value greater than the critical value of -0.055 (that is, -0.054 or higher) in the nucleation region, giving a d/p value equal to three times that in the areas of the liquid crystal layer having a thickness of 3gm. As a result, when no voltage is applied across the liquid crystal layer the 70' twist state will be stable in the nucleation region, whereas the 7r)' 110' twist state will be stable in the active region. When a voltage is applied across the liquid crystal layer, the = 700 twist state in the nucleation region will propagate into the active region, and displace the ( - n)' twist state.

The present invention guarantees the nucleation of the = 700 twist state on application of a suitable voltage to the liquid crystal layer, by providing the nucleation region with a higher value of d/p than (d/p), owing to the smaller thickness of the nucleation region compared to that of the active region.

A further application of the present invention is to a reflective display device contains a micro-reflective structure (MRS). An MRS consists of a smoothly varying surface relief that is coated with a reflective material such as aluminium. This surface relief may be employed to provide nucleation regions in the device.

As an example, the super mobile HR TFT reflective LCD produced by Sharp Corporation contains a MRS that is provided on the rear substrate of the device adjacent to the liquid crystal layer. The effect of providing the MRS is to cause the thickness of 19 the liquid crystal layer to vary between 2Aptin and 3.6gin. In order to provide a nucleation region, the value of the d/p ratio of the liquid crystal material should be set greater (less negative) than (d/p)c at a liquid crystal layer thickness of 2.4PLm. This will give a value of d/p which is about 1.25 more negative than (d/p)c at the nominal thickness of the display (3gm). As described above, when a voltage is applied across the liquid crystal layer the = 70' twist state, which is the stable state in the nucleation region, will then propagate into the active region of the display device.

Claims

CLAIMS:

1. A liquid crystal display device comprising: a layer of a chiral liquid crystal material disposed between first and second substrates; and means for applying a voltage across the liquid crystal layer; wherein a first region of the liquid crystal layer is an active region for display and a second region of the liquid crystal layer is a nucleation region for generating a desired liquid crystal state in the first region when a voltage is applied across the liquid crystal layer, the first region of the liquid crystal layer having a first thickness dA and the second region of the liquid crystal layer having a second thickness dN different from the first thickness.

2. A liquid crystal display device as claimed in claim 1, wherein the second thickness is selected such that, when no voltage is applied across the liquid crystal layer, the liquid crystal state stable in the second liquid crystal region is topologically equivalent to the desired liquid crystal state.

3. A liquid crystal display device as claimed in claim 1 or 2, wherein the first and second thicknesses are selected such that, when no voltage is applied across the liquid crystal layer, a first liquid crystal state having a first twist angle is stable in the first liquid crystal region, and a second liquid crystal state having a second twist angle different from the first twist angle is stable in the second liquid crystal region.

4. A liquid crystal display device as claimed in claim 1, 2 or 3 wherein the twist of the liquid crystal layer is in the same sense as the natural twist of the liquid crystal material and the second thickness is greater than the first thickness.

5. A device as claimed in claim 4 wherein the first liquid crystal state is a 00 twist state and the second liquid crystal state is a 180' twist state.

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6. A liquid crystal display device as claimed in claim 4 or 5 wherein the alignment direction on the first substrate is parallel to the alignment direction on the second substrate, and the first and second thicknesses are selected such that dAlp < 0.25 and 0.25:5 dN/P:5 0.75, where p is the pitch of the liquid crystal material.

7. A liquid crystal display device as claimed in claim 6 where dA/P < 0. 125.

8. A liquid crystal display device as claimed in claim 6 where dA/p < 0. 1.

9. A device as claimed in any of claims 4 to 8 wherein the 0' twist state is an H- state.

10. A device as claimed in any preceding claim wherein the device is a surface mode liquid crystal display device.

11. A device as claimed in claim 10 wherein the device is a pi-cell.

12. A device as claimed in claim 11 wherein the desired liquid crystal state in the active region is a V-state.

13. A device as claimed in any preceding claim wherein the device is a reflective liquid crystal display device.

14. A device as claimed in claim 13 wherein the second liquid crystal layer region is disposed at a via-hole.

15. A device as claimed in claim 1 or 2 wherein the twist of the liquid crystal layer in the second region of the liquid crystal layer is nonzero and is opposite to the natural twist of the liquid crystal molecules, and the second thickness is smaller than the first thickness.

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16. A device as claimed in claim 15 wherein the alignment direction on the first substrate is at an angle to the alignment direction on the second substrate, and the first and second thicknesses are selected such that:

d4 < 0 _1; and p 2.z 4 dN > 0 - 1. p 2x 4 ' where p is the pitch of the liquid crystal material.