WO2002067045A1

WO2002067045A1 - A method of changing the phase of bistable cholesteric liquid crystals

Info

Publication number: WO2002067045A1
Application number: PCT/SE2002/000257
Authority: WO
Inventors: Stephen Palmer
Original assignee: LC-TEC SWEDEN AB
Current assignee: LC-TEC SWEDEN AB
Priority date: 2001-02-23
Filing date: 2002-02-14
Publication date: 2002-08-29
Anticipated expiration: 2003-08-23
Also published as: SE0100624D0; SE519582C2; SE0100624L

Abstract

A method of changing the liquid crystal phase of bistable, cholesteric liquid crystals in a display is provided. In the display there is provided at least two active display areas with inactive display areas lying between said active display areas, and a D.C voltage is applied for a predetermined period of time to neighbouring active display areas, thereby causing the liquid crystal phase of the inactive display areas between the active areas to changed to a predetermined state. In such way, mechanical pressure marks in cholesteric liquid crystal displays can be removed. A display adapted for this method is also disclosed.

Description

A method of changing the phase of bistable cholesteric liquid crystals

FIELD OF INVENTION

The present invention relates generally to a method of driving liquid crystal displays and more specifically to a method of changing the liquid crystal phase of bistable, cholesteric liquid crystals in a display.

BACKGROUND OF THE INVENTION

A wide range of electronic display technologies have been developed over the years. One of these technologies is the so called Liquid Crystal Display (LCD) technology that utilises liquid crystal materials bounded between two parallel substrates. LCD's are light weight and consume little electrical power in comparison to other electronic display technologies. The most commonly used LCD technology is the so-called Twisted-Nematic (TN) technology often incorporated in standard displays for watches and calculators.

Another type of LCD technology is the cholesteric liquid crystal display technology that utilises cholesteric liquid crystal materials , otherwise known as chiral twisted-nematic liquid crystal materials. By using state of the art techniques, cholesteric liquid crystal materials can be made to be bistable, meaning that there can be at least two cholesteric liquid crystal phases that are stable at or near to room temperature in the absence of an externally applied voltage. Switching between said stable phases occurs typically via application of a suitable electrical voltage pulse. Two state of the art techniques that induce bistability in the liquid crystal phases of a cholesteric liquid crystal material are the Surface Stabilised Cholesteric Textured (SSCT) technique, and the Polymer Stabilised Cholesteric Textured (PSCT) technique. The former named technique uses a surface alignment material in order to stabilise the cholesteric liquid crystal phases at or near to room temperature, whereas the latter technique makes use of a polymer network in order to induce stability in said cholesteric liquid crystal phases.

Two such stable phases at or near to room temperature are the focal conic texture, which is predominately transparent to visible light radiation, and the planar texture, which can be coloured when viewed in reflection. By paint- ing the back of the display with a dark mask, the focal conic texture will appear dark. This gives the possibility of manufacturing cholesteric liquid crystal displays where individual areas of the display can be either switched to a dark state, where the liquid crystal material is in the focal conic texture and the dark mask is observed, or to a reflecting coloured state, where the liquid crystal material is in the planar texture.

Switching of the cholesteric liquid crystal material between said bistable phases at or near to room tempera- ture typically occurs via use of state of the art electrical voltage pulses. A high voltage pulse, see fig. la, typically switches the cholesteric liquid crystal material to the reflecting planar phase, whereas a lower voltage pulse, see fig. lb, typically switches the material to the focal conic texture. Prior art has also demonstrated that the rate of voltage discharge across the cholesteric liquid crystal material also affects the phase to which said material switches. For example, with an initial high voltage acting over the cholesteric liquid crystal material such that said material is switched to the homeotropic phase, characterised by the dielectric anisotropy of said material being predominately aligned parallel with the electrical field, a subsequent slow discharge of the voltage, as shown in fig. lc, will result in said liquid crystal material being switched to the focal conic texture. Conversely, a rapid discharge of said voltage will switch said material to the planar phase, shown in figure la.

Cholesteric liquid crystal displays can be produced using state of the art techniques whereby the passive matrix driving scheme is employed. Figures 2 and 3 show the preferred row and column electrode configuration according to prior art in a passive matrix liquid crystal display. Here, a series of parallel rows 2 of electrodes with gaps 4 therebetween are formed on one side of the liquid crystal cell, and a series of parallel columns 6 of electrodes with gaps 8 therebetween are formed on the other side of the liquid crystal cell such that the rows are predominately mutually perpendicular to the columns . The inter- sections of row electrodes on one side of the cell with column electrodes on the other side of said cell defines the picture elements or pixels 10 of the display, shown in fig. 3.

By applying state of the art voltage pulses as shown in figs, la-c in sequence to the individual rows and columns of a passive matrix display, a picture or image can be scanned into the display row-by-row, whereby the cholesteric liquid crystal material at each pixel location 10 is switched to form either the focal conic or planar texture. Once an image has been scanned into the pixels of a chole- steric liquid crystal display, the bistability of the cholesteric liquid crystal material results in no electrical power being required in order to maintain the image.

The liquid crystal material in the regions between the pixels , hereinafter referred to as the inter-pixel regions 12, is not bounded on each side by electrodes, hence it is not possible to directly activate the cholesteric liquid crystal material in said regions when using state of the art technology. The inter-pixel regions are therefore static or inactive and hence it is desirable to minimise the area of said regions in order to improve the optical properties of the display.

Prior art has demonstrated that if the inter-pixel spacing is less than approximately twice the gap between the parallel substrates in the liquid crystal cell, it is possi- ble to generate an interstitial voltage in the inter-pixel region by applying either alternating current (A.C), or direct current (D.C) voltage to neighbouring pixel electrodes. In such case, the interstitial voltage interacts with the cholesteric liquid crystal material in the inter- pixel region, making it possible to electrically switch said liquid crystal material to the required liquid crystal phase. This effect occurs when either A.C or D.C voltage is applied to neighbouring pixel electrodes.

For example, with a liquid crystal cell possessing a cell gap of 5 microns, an inter-pixel spacing of less than 10 microns is required for said effect to occur. However, from manufacturing considerations, the preferred inter- pixel spacing lies between approximately 10 microns and 200 microns for a passive matrix, cholesteric liquid crystal display. With inter-pixel spacings of less than approximately 10 microns, there is an increased risk of defects, giving rise to electrical short circuits between adjacent pixels due to manufacturing tolerances. The named preferred inter-pixel spacing results in a large total area of the cholesteric liquid crystal display being inactive .

A further problem of state of the art cholesteric liquid crystal displays is that they are pressure sensitive. Application of mechanical pressure to the display surface results in lateral fluid flow of the cholesteric liquid crystal material around said pressure point. This fluid flow disrupts the bistable textures of the cholesteric liquid crystal material and hence changes the optical appearance of the display around said point.

When a new image is scanned into a cholesteric liquid crystal display possessing one or more pressure marks, the liquid crystal material at each pixel point is switched to the required state by the externally applied voltage pulses. However, the cholesteric liquid crystal material in the inter-pixel regions is not assessable with state of the art voltage pulses and when the preferred inter-pixel spacing of between 10 microns and 200 microns is used, hence the disruption in the liquid crystal material texture due to the application of mechanical pressure remains in said regions and is manifested as an optical defect. In order to help prevent the formation of mechanical pressure marks in cholesteric liquid crystal displays, a number of prior art techniques have been suggested. One method involves the formation of a rigid polymer network in the cholesteric liquid crystal material that helps to geographically localise said liquid crystal material and hence minimise the lateral flow upon application of mechanical pressure to the display surface. Another technique involves the formation of rigid distance spheres or spacers between the two parallel substrate plates of the liquid crystal cell . This helps to minimise the change in the cell gap upon application of mechanical pressure. However, neither technique satisfactorily alleviates the problem of pressure sensitivity and further complicates the liquid crystal display manufacturing processes.

Prior art has demonstrated that said pressure marks in bistable, cholesteric liquid crystal displays can be thermally removed by heating part or all of the display such that the cholesteric liquid crystal material changes state to form the isotropic phase, characterised by there being no long range molecular stacking structure present in the liquid crystal material. Subsequent cooling of the display causes the cholesteric liquid crystal material to change phase, forming one of the bistable textures when at or near to room temperature. This results in the entire cholesteric display being thermally reset and all prior pressure marks as well as any previously held images are removed. Although being functional, such thermal processes are not preferred when considering a liquid crystal dis- play product suitable for the consumer market. The patent document US 4,212,010 A (Walter) discloses a method for the operation of a display device having a bistable liquid crystal layer. The problem of removing pressure marks is not addressed in this document.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method to remove mechanical pressure marks in cholesteric liquid crystal displays, whereby the cholesteric liquid crystal material in both the pixel and inter-pixel regions is switched to one of the bistable cholesteric liquid crystal phases.

The invention is based on the insight that, despite the common prejudice that the use of D.C voltage for the driving of liquid crystal displays is usually avoided due to the risk of ion migration within, for example, the liquid crystal material, mechanical pressure marks can be electronically removed by application of a high, D.C voltage to all or some of the active display areas simultaneously for extended periods of time .

According to the invention, there is provided a method as defined in appended claim 1. There is also provided a display as defined in claim 6.

With the inventive method and display, the drawbacks of prior art methods and devices are avoided or at least mitigated. A method is provided, wherein mechanical pressure marks can be removed at or near to room temperature by means of conventional control electronics, thus providing a simple and inexpensive cholesteric liquid crystal display. BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood and its objects and advantages will become apparent to those skilled in the art by reference to the accompanying drawings wherein like reference numerals refer to like elements in several of the figures and in which:

Figs . la-c are curve diagrams showing prior art voltage pulses used for switching cholesteric liquid crystal materials;

Fig. 2 is a perspective view of a row and column electrode configuration according to prior art in a passive matrix liquid crystal display;

Fig. 3 is a plan view of a row and column electrode configuration according to prior art in a passive matrix liquid crystal display.

Fig. 4 is a cross section of a liquid crystal cell according to the invention;

Fig. 4a is an enlarged view of a portion of the liquid crystal display shown in fig. 4; and

Figs. 5a and 5b show an electrode configuration for a 7 segment numerical cholesteric liquid crystal display according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

A preferred embodiment of the method and the display according to the invention will now be described, mainly with reference to fig. 4, which is a cross-sectional view of a liquid crystal cell construction or liquid crystal display, generally designated 1. The display 1 comprises a first generally planar side or substrate plate 14a and a second generally planar side or substrate plate 14b at a uniform mutual distance d. Liquid crystal material in the form of bistable, cholesteric liquid crystals (for the sake of clarity not shown in the figure) are provided between the two plates 14a and 14b.

Provided on the first plate 14a are a number of row electrodes 2, two of which are shown in the figure. The row electrodes 2 shown in the figure are separated by a gap 4 constituting part of an inter-pixel region 12 similar to those shown in fig. 3. The second plate 14b comprises a column electrode 6 provided in a direction perpendicular to the direction of the row electrodes 2 , as shown in figs. 2 and 3.

Application of a voltage to the row and column electrodes 2 and 6 in a passive matrix liquid crystal display shown in fig. 4 generates an electric field 16 between the two substrate plates across the liquid crystal material. At the edges of the pixel regions, one of which is shown in detail in fig. 4a, the electric field extends into the inter-pixel area or gap 4. The extension of the electric field perturbation into said regions is of the order of several microns and is, amongst other things, a function of the liquid crystal cell gap, d.

If the inter-pixel spacing 12 in figs. 2 and 3 is less than approximately twice the distance d between the liquid crystal substrate plates 14a and 14b in fig. 4, the extension of the electric field 16 at the edges of neighbouring pixels converge and hence is able to activate the liquid crystal material present in the inter-pixel regions. For example, with a typical liquid crystal cell gap of 5 microns, the inter-pixel spacing is required to be less than approximately 10 microns for said effect to occur. This effect occurs when either A.C or D.C voltage is used.

However, due to manufacturing tolerances, the preferred inter-pixel spacing lies between 10 microns and 200 microns. In such case, the lateral component of the perturbed electric field at the edges of the pixels generates a lateral force on charge carriers present in the row and column electrodes, stimulating charge migration into the inter-pixel regions through the various liquid crystal cell layers.

When D.C voltage is applied to the row and column elec- trodes for prolonged periods of time, the process of charge migration such as, for example, electron hopping, into the inter-pixel region through the cell layers is allowed to proceed, resulting in the local accumulation of charges in said regions. This enables the voltage applied to the electrodes of neighbouring pixels to creep into the inter-pixel areas . Once the inter-pixel voltage has been established, the cholesteric liquid crystal material in both the inter-pixel and pixel regions is electronically switched to the homeotropic phase and the inter-pixel activation state is attained.

Removal of the externally applied D.C voltage to some or all of the pixel electrodes sets the cholesteric liquid crystal material in the pixel regions to either the focal conic or planar textures, depending upon the rate of dis- charge of the local voltage in said areas. This also allows for the relatively slow discharge of the inter- pixel voltage through the liquid crystal cell layers with a time period extending to several seconds, hence the cholesteric liquid crystal material in the inter-pixel regions is set to the focal conic texture.

For said effect to occur, one or more layers are required in the liquid crystal cell construction that permit the relatively slow passage of charges from the pixel electrodes into the inter-pixel regions through said layers, and which prevent said charges from spontaneously leaking away through the liquid crystal material, allowing for the accumulation of local charges in said region and hence the build-up of voltage in the inter-pixel area.

Figs. 4 and 4a show one liquid crystal cell construction that satisfies these requirement. To this end, two thin films are stacked on top of each other and coat the two inner surfaces of the liquid crystal cell, shown in figure 4. The first thin film layer 18 is a hard-coat material, such as AT-720A from Nissan Chemicals. The thickness of the hard-coat layer is approximately lOOnm and the material has a relatively low resistivity of approximately 10¹⁴ ohms-cm. The hard-coat thin film is in electrical contact with the electrodes 2 and 6 and due to the relatively low resistivity of this layer, charges 22 are able to pene- trate through said layer and hence migrate into the inter- pixel region upon application of a D.C voltage for extended periods of time to the electrodes of the neighbouring pixels, as indicated by the arrow designated F_migration in fig. 4a. The second thin film 20 coats the top of the first said hard-coat layer and can be a polyimide material, such as SE-7511L from Nissan Chemicals. This layer has a thickness of approximately lOOnm and the relatively high material resistivity of approximately 10¹⁶ ohms-cm prevents said charges in the inter-pixel regions from leaking away through, for example, the liquid crystal material.

Elevated temperatures further reduce the resistivity of the low resisitivity layer and hence increases the rate of charge migration from the edges of the pixel electrodes into the inter-pixel regions under action of a D.C voltage. Higher applied D.C voltages also increases the rate of voltage creepage into the inter-pixel regions . Reduced inter-pixel spacings lowers the required distance for charge penetration. All the above named effects reduce the time period required in order to build up the inter-pixel voltage and hence attain the inter-pixel activation state .

D.C voltage is required in order to generate said inter- pixel voltage and this effect does not occur when A.C voltage is used. Application of D.C voltage to a cholesteric liquid crystal cell also has advantages over that of A.C voltage in terms of the current consumption of the display. The use of D.C voltage for the driving of liquid crystal displays is usually avoided due to the risk of ion migration within, for example, the liquid crystal material over prolonged periods of time. This can result in the build-up of charges in the liquid crystal cell layers, generating image sticking problems and hindering cell switching. However, when a bistable liquid crystal mate- rial is used, voltage is only required when the displayed image is changed or when the liquid crystal display is to be electronically reset. For the remaining period of time, no voltage is applied to the liquid crystal cell. This allows for the diffusion and dissipation of any charges that have built-up during the relatively short periods of time when D.C voltage is applied (several tens of seconds ) . The problem of ion migration in a liquid crystal cell under the action of D.C voltage is therefore essentially alleviated via use of a bistable liquid crystal material.

The time required in order to attain the inter-pixel activation state and hence electronically reset a cholesteric liquid crystal display depends, amongst other things, upon the magnitude of the applied D.C voltage. Typical values are approximately 30 seconds with a D.C voltage of 80 volts and 45 seconds with a D.C voltage of 40 volts at room temperature and with an inter-pixel spacing of 70 microns . These times are temperature sensitive and elevated temperatures dramatically reduce the required time period. For example, at a temperature of 60°C, only approximately 5 seconds is required in order to reset a cholesteric liquid crystal display when using a D.C voltage of 40 volts and with the above mentioned inter-pixel distance.

The size of the inter-pixel gap 12 in fig. 3 also affects the time required in order to generate the inter-pixel voltage when a D.C voltage is applied to all or some of the row and column electrodes simultaneously. In general, the smaller the inter-pixel gap, the shorter is the time required to reset the cholesteric liquid crystal display. The above described novel effect of the use of D.C voltages is specifically, but not limited, to a cholesteric liquid crystal display using the state of the art passive matrix driving scheme, whereby there is a matrix of active pixel areas characterised by the liquid crystal material in said regions being bounded by electrodes on both sides of the cell, and with static, inactive regions of display areas lying between said active pixels , as described above with reference to figs. 2-4. The present invention is also applicable to other liquid crystal display electrode configurations, whereby the active display regions or segments form, for example but not exclusively, numerical, alphanumerical or pictorial characters .

Figs. 5a and 5b demonstrate a first side 114a and a second side 114b, of an electrode configuration for a cholesteric liquid crystal display 101 showing a seven segment numerical character. Here, an outside electrode 118 is also included that surrounds and encapsulates the seven active areas 102a-f making up the numeral. Each of the active areas is controlled by means of a respective control electrode 104a-g. Application of a D.C voltage for prolonged periods of time simultaneously to the contact electrodes of the active segments results in a build up of voltage in the inactive areas bounded by said active segments. This results in the cholesteric liquid crystal material in both the active and inactive regions being electronically switched to the homeotropic phase, characterised by the dielectric anisotropy of the cholesteric liquid crystal material being aligned predominantly parallel with the electric field. Subsequent removal of the applied electric field enables the slow discharge of the voltage in the areas bounded by neighbouring active segments and the cholesteric liquid crystal material in said regions is hence switched to the focal conic texture. This provides for the electrical removal of prior mechanical pressure marks present in the cholesteric liquid crystal material in both the active and inactive regions and the display is hence electronically reset where all previously held images are erased. This procedure can be repeated automatically with predetermined intervals, such as once every day.

While a preferred embodiment has been shown and described, various modifications may be made thereto without departing from the inventive idea of the present invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation.

It will be obvious to one skilled in the art that other liquid crystal cell constructions are also possible that allow for the migration of charges from the pixel electrodes into the inter-pixel regions under application of a D.C voltage, and which prevent or delay the spontaneous leakage and dissipation of said charges through the liquid crystal material, hence allowing for the accumulation of local charges in said regions and the build-up of the inter-pixel voltage.

The term D.C voltage is intended to cover all voltages where the sense of polarity of the voltage is unchanged over an extended period of time, although the magnitude of the voltage may change throughout said time period.

Claims

1. A method of changing the liquid crystal phase of bistable, cholesteric liquid crystals in a display, comprising the steps of:

- providing at least two active display areas with inactive display areas lying between said active display areas , and

- applying a D.C voltage for a predetermined period of time to neighbouring active display areas,

characterized in that

the step of applying a D.C. voltage results in switching the liquid crystal phase of bistable, cholesteric liquid crystals of an inactive display area between said neighbouring active display areas to a predetermined state.

2. The method according to claim 1, wherein said predetermined state is the focal conic texture.

3. The method according to claim 1 or 2, wherein the step of applying a D.C voltage results in the removal of mechanical pressure marks in said inactive regions.

4. The method according to any of claims 1-3, wherein the step of applying a D.C voltage comprises the step of applying a D.C voltage for prolonged periods of time to some or all of said row and column electrodes simultaneously.

5. The method according to any of claims 1-4, wherein the step of applying a D.C voltage comprises applying a D.C voltage having a voltage between 5 and 200 volts, and more preferably between 10 and 100 volts and being applied to said active display elements for a time period of between 0,5 and 500 seconds, and more preferably between 0,5 and 200 seconds.

6. A display of bistable, cholesteric liquid crystals, comprising:

- a series of row electrodes (2) on a first side of a liquid crystal cell,

a series of column electrodes (6) on a second side of said liquid crystal cell,

at least two active display areas (10) with inactive display areas ( 12 ) lying between said active display areas, wherein an intersection of a row electrode and a column electrode defines one of said active display areas , and

- electronic circuitry arranged to apply a D.C voltage for a predetermined period of time to neighbouring active display areas,

characterized in that

the distance between two adjacent row or column electrodes is less than approximately twice the distance (d) between said row electrodes and said column electrodes, whereby applying a D.C voltage results in switching the liquid crystal phase of an inactive display area to a predetermined state.

7. The display according to claim 6, comprising:

a first layer (18) of material with relatively low resistivity, preferably of approximately 10¹⁴ ohms-cm, in electrical contact with said electrodes, and

- a second layer (20) of material with relatively high resistivity, preferably of approximately 10¹⁶ ohms-cm, in electrical contact with said first layer.

8. The display according to claim 6 or 7 , wherein said at least two active display areas comprises the electrodes forming numerical, alphanumerical or pictorial active display segments separated by gaps, and wherein said gaps form said inactive display areas.

9. The display according to any of claims 6-8, wherein said active display areas are separated by a distance of between 10 microns and 200 microns.