WO2010013041A1

WO2010013041A1 - Liquid crystal displays

Info

Publication number: WO2010013041A1
Application number: PCT/GB2009/050932
Authority: WO
Inventors: Christopher James Newton Fryer; Christopher Miles Evans; Yong Zhao; William Frank Tyldesley
Original assignee: Pelikon Limited
Priority date: 2008-07-31
Filing date: 2009-07-28
Publication date: 2010-02-04
Also published as: CN102112908A; EP2307925A1; GB0813970D0

Abstract

This invention concerns a method of manufacturing a display and a display. The method comprises providing a substrate (11, 111) having a first electrode (12, 112) formed thereon and coated with physically stabilised liquid crystal layer (13) and printing a second transparent electrode (20) over the physically stabilised layer (13). A dielectric layer (119) may be formed between the physically stabilised liquid crystal layer (13) and the second electrode (20), preferably having a dielectric strength of more than 2V/μm. A barrier layer (110) may be provided to restrict migration of liquid crystal from the liquid crystal layer. The method may comprise performing a wash of the second electrode to remove corrosive materials left over from printing of the second electrode.

Description

LIQUID CRYSTAL DISPLAYS

This invention is concerned with liquid crystal displays .

In known liquid crystal displays, a liquid crystal shutter (LC) , comprising a liquid crystal layer sandwiched between a pair of transparent electrodes, is placed in front of a backlight, such as an electroluminescent backlight, such that the backlight is rearward of the rear electrode of the LC shutter and illuminates the LC shutter. Application of a voltage to the electrodes causes switching of the liquid crystal layer between a transmissive state, in which light from the backlight can pass through the LC layer to the viewer, and an absorbing or scattering state, in which the LC layer blocks light emitted by the backlight from reaching the viewer.

It is known for the LC layer to be in the form of a physically stabilised liquid crystal layer, such as a polymer dispersed liquid crystal (PDLC) in which droplets of liquid crystal, typically nematic or cholesteric in nature, are dispersed in a polymer matrix (binder) .

PDLC shutters are manufactured by forming a transparent conductive layer, usually a layer of ITO material, on a substrate to form a front electrode for the PDLC shutter, coating a PDLC layer on the transparent electrode and then laminating a further substrate also having a conductive layer, usually of ITO material, to the PDLC layer to form a rear electrode.

It will be understood that "front" as used herein means the side of the display from which the display is to be viewed.

It is desirable to produce thinner LC shutters than the shutters produced by the above method at a lower cost. An alternative to an LC shutter is described in International patent application No: WO 2005/0121878, International patent application No: WO2008075001 and UK patent application No: 0865751.5. These patent applications describe an electroluminescent display (Hybrid Display) with an EL layer and a Liquid Crystal (LC) mask that are switchable in individual areas to define information to be displayed, The hybrid display differs from the conventional LC shutters in that the LC mask and the EL layer are formed as a single integral unit wherein both components (the LC mask and EL layer) are operated using common electrodes - the EL layer mounted between the layer of liquid crystal material and the rear electrode. The front electrode is typically a layer of ITO material and the rear electrode is a printed electrode, typically formed from an opaque carbon filled ink.

According to a first aspect of the invention there is provided a method of manufacturing a display comprising providing a substrate having a first electrode formed thereon and coated with a physically stabilised liquid crystal layer and printing a transparent second electrode over the physically stabilised layer.

According to a second aspect of the invention there is provided a display comprising a physically stabilised liquid crystal layer between a first electrode and a printed, transparent second electrode.

The second electrode may be formed from a polymer mixture of a sulphonated polystyrene and a polythiophene based polymer.

It has been found that a polymer mixture of a sulphonated polystyrene and a polythiophene based polymer produces a second electrode that is transparent and printable and can match the performance of non-printed electrodes, such as ITO.

Preferably, the sulphonated polystyrene carries a negative charge, generally arrived at by deprotonation of part of the sulphonyl groups and, in one embodiment, is sodium polystyrene sulphonate.

Preferably, the polythiophene based polymer is a conjugated polymer and more preferably, carries a positive charge. The polythiophene based polymer may be a polyethylenedioxythiophene and most preferably, 3,4- ethylenedioxythiophene .

The polymer mixture may be a macromolecular salt where both components are charged.

The material of the second electrode may have a transparency above 70% and haze below 20%, preferably below 11%. For example, the polymer mixture may be PEDOT: PSS.

The method may comprise forming a barrier layer between the physically stabilised layer and the second electrode, the barrier layer arranged to restrict migration of liquid crystal from the physically stabilised liquid crystal layer to layers rearward of the barrier layer.

It has been found that printing the second electrode over the physically stabilised layer rather than laminating the second electrode onto the physically stabilised layer creates a thinner construction and, potentially, reduces costs as the formation of the second electrode over the physically stabilised layer can be carried out as a single stage process (printing) rather than a multi-stage process (forming the second electrode on a substrate and then laminating the substrate with the second electrode to a substrate with the first electrode and physically stabilised layer) . Printable transparent materials that can be used to form the second electrode tend not to be effective barriers to the migration of liquid crystal for the LC layer. Accordingly, to form a LC display by printing the second electrode it may be necessary to provide a barrier layer to restrict the migration of liquid crystal from the liquid crystal layer to lengthen the life of the display. Without a barrier layer, liquid crystal may migrate to the second electrode's surface and be evaporated therefrom.

The display may comprise an LC shutter, the LC shutter comprising the physically stabilised liquid crystal layer, the first electrode, the second electrode and the barrier layer. The display may comprise a backlight rearward of the second electrode for illuminating the LC shutter. An LC shutter does not comprise an EL layer that is driven by the first and second electrodes (common electrodes) that are used for switching the LC layer.

Alternatively, the display may comprise a hybrid display arrangement comprising an EL layer behind the LC layer that is driven together with the LC layer by the first and second electrodes.

Liquid crystal vesicles of the physically stabilised layer may include a dye and the barrier layer may be arranged to limit the migration of liquid crystal together with the dye from the LC layer to layers behind the LC layer such that no significant fading of the display occurs due to migration of the liquid crystal together with the dye under pre-determined criteria. It will be understood that the term "significant fading" is used herein to mean changes in contrast of the display that are noticeable to the naked eye, The barrier layer may be arranged to limit the migration of liquid crystal from the LC layer to layers behind the LC layer such that, when the display is heated to 85 degrees over 18hrs, there is virtually no change, and preferably no change whatsoever, in contrast between illuminated and non-illuminated areas of the display.

The barrier layer may comprise a material in which the liquid crystal has low solubility. As liquid crystal in the LC layer cannot dissolve in the barrier layer, migration of liquid crystal through the barrier layer to layers behind the LC layer is reduced or even eliminated.

The barrier layer may comprise a material in which the liquid crystal has solubility lower than the solubility of liquid crystal in the polymer resin used in layers behind the barrier layer, such as the phosphor, dielectric, insulator and/or second electrode layers . In particular, the barrier layer may comprise material in which the liquid crystal has a low enough solubility such that no significant fading of layers behind the LC layer occurs due to migration of the liquid crystal under pre-determined criteria, for example, no significant fading of the layers behind the LC layer occurs due to migration of the liquid crystal when the display is heated at a set temperature for a set time, for example 85⁰C for approximately 18 hrs.

Solubility can be measured as the maximum amount of solute that can dissolve per amount of solvent under specified conditions. The barrier layer may comprise a material in which the liquid crystal has substantially zero, and preferably zero, solubility at room temperature and atmospheric pressure.

The barrier layer may comprise a hydrophilic layer, in particular a hydrophilic polymer. The polymer may be a water soluble polymer, such as polyvinyl alcohol, polyvinyl butyral, polyethylene glycol, polypropylene glycol, gelatine and its derivatives, cellulose derivatives polyacrylics and derived polymers and polyacrylic acids and derived polymers . Alternatively, the polymer may be a non- water soluble polymer that can be carried in an emulsion and/or dispersion, such as polyurethanes, polyethylene-acylic acid copolymer and derived copolymers, polymethacrylics and derived copolymers and polymethacrylic acids and derived copolymers. Preferably, the polymer is a polyvinyl alcohol (PVA) .

Furthermore, the barrier layer may comprise an adhesion promoter to improve the adhesion of the barrier layer with the LC layer. The barrier layer may further comprise a surfactant.

When a hydrophilic barrier layer absorbs water it increases its electrical conductivity (reduces its resistivity) such that the electrical characteristics of the display may be similar to such a display without the barrier layer. For example, a completely dry hydrophilic polymer layer may have a resistance of about 10¹⁰ Ωcm, whereas, after absorbing water from the environment, its resistivity may reduce to 10⁶ or even 10⁵ Ωcm. In this way, the introduction of a hydrophilic barrier layer has little, or even no, impact on the electrical characteristics of the display, but acts to prevent migration of the liquid crystal.

It is difficult to produce a physically stabilised LC layer that is free from small holes, known as pin-holes, of a typical diameter of lOOμm or less. In the laminated construction of the prior art, these holes do not present a problem as the ITO material is not capable of forming a bridge through the hole to cause a short circuit. However, with a printed second electrode, the ink that forms the second electrode can flow into these pinholes and cause a short circuit, in use. Accordingly the method may comprise forming a dielectric layer over the physically stabilised LC layer and printing a second electrode over the physically stabilised liquid crystal layer and the dielectric layer. This will produce a display comprising a dielectric layer between the physically stabilised liquid crystal layer and the second electrode.

The display may be an LC shutter and the method may comprise forming a dielectric layer over the physically stabilised LC layer and printing the second electrode over the physically stabilised liquid crystal layer and the dielectric layer.

The dielectric layer may have a dielectric strength of more than 2V/μm, and preferably 20V/μm,

The dielectric layer acts as a barrier to prevent the ink of the second electrode from entering the pin-holes in the physically stabilised layer during manufacture and therefore, reduces the chance of a defective display being manufactured.

The dielectric layer may be transparent so that the LC shutter can be used with a backlight. Accordingly, in one embodiment, the display comprises a backlight rearward of the second electrode.

In an alternative arrangement, the dielectric layer is reflective. Such an LC shutter works by reflecting the ambient light in regions of the LC layer that have been switched into a transparent state.

In an arrangement wherein the dielectric layer is reflective, it may not be necessary for the rear electrode to be transparent. Accordingly, in a third aspect of the invention a method of manufacturing a display comprises providing a substrate having a first electrode formed thereon and coated with a physically stabilised liquid crystal layer, forming a dielectric layer and printing a second electrode such that the dielectric layer is between the physically stabilised liquid crystal layer and the second electrode, the dielectric layer having a dielectric strength of at least 2V/μm,

According to a fourth aspect of the invention there is provided a display comprising a physically stabilised liquid crystal layer between a first electrode and a printed, second electrode and a dielectric layer between the physically stabilised liquid crystal layer and the second electrode, the dielectric layer having a dielectric strength of at least 2V/μm.

The dielectric layer may be printed directly onto the physically stabilised liquid crystal layer.

Preferably, the dielectric layer is relatively thin having a thickness of less than 25μm and preferably around 5μm. A thin dielectric layer ensures that, in use, the dielectric layer does not significantly reduce the electric field applied across the LC layer. Furthermore, it is desirable that the dielectric layer has a high dielectric strength of greater than 2V/μm, preferably 20 V/μm to be able to withstand in excess of 100V across 5μm in the region of pin-holes. Additionally, in order to reduce the field that appears across this layer in the regions away from the holes, the material of the dielectric layer should have a high dielectric constant, for example greater than 3 and preferably, above 5. Typical values would be 8 to 15.

Appropriate materials for the dielectric layer include polyvinylidene fluoride (PVDF) , polyvinylidene fluoride/hexafluoropropylene

(PVDF/HFP) , polyester, vinyl, epoxy or polyvinylidene fluoride copolymers, such as Kynar 9301 , a proprietary terpolymer sold by Atofina, and cyano-resins, a range of proprietary polymers sold by Shin- Etsu Chemical Company. Cyano resins are produced by the reaction of acrylonitrile with a polymer having hydroxyl functionality.

The first substrate may be a transparent substrate and the first electrode comprising a transparent conductive layer, for example such as an indium tin oxide layer (ITO) .

Even though by choosing an appropriate material for the second electrode may reduce the chance of a short circuit, problems can still result from the interaction of components of the ink used to form the second electrode with the first electrode through the pin-holes. For example, PEDOT: PSS, such as that formed by BAYER as BAYTRON or sold as an ink by AGFA GEVAERT as ORGACON, is a suitable material for the second electrode. The PEDOT conducting polymer in these materials is stabilised in a solid dispersion using PSS (polystyrene sulphonate) . The PSS component is acidic and, in use, the second electrode can pass through the pin-holes and attack the material, such as ITO, of the first electrode. This attack on the ITO layer can result in large and extended regions of non- operation. It is understood that BAYTRON and ORGACON formulations contain an excess of PSS in order to improve dispersion stability and therefore, this problem is particular pronounced with these materials.

Accordingly, the method may comprise performing a wash, preferably an aqueous wash, of the second electrode. Preferably, the washing fluid is water, although the fluid may include small quantities of additives such as surfactants and detergents to improve the efficiency of the washing process. It has been found that the wash removes corrosive materials left over from printing of the second electrode, such as excess PSS, reducing or even preventing acidic erosion of the first electrode through pin-holes in the PDLC layer.

The aqueous wash may be carried out in by washing the second electrode in a static bath two or more times or by allowing water to continuously flow over the second electrode for a predetermined length of time. The important feature of the wash is that it removes sufficient amounts of corrosive materials left over from printing of the second electrode in order to achieve a level of reliability in the manufacturing process. Accordingly, the approach taken to the wash will depend on how much of the corrosive materials it is deemed appropriate to remove, which itself will depend on the quality of the display to be produced and the level of reliability required in the manufacturing process. It has been found that soaking the film in water for an hour may remove substantially all of the excess PSS when printing an electrode made of PEDOT: PSS.

The liquid crystal layer is in a physically stabilised form rather than its normal liquid mobile form, for example the liquid crystal may be liquid crystal vesicles encapsulated in a polymer matrix, such as a polymer- dispersed liquid crystal (PDLC) , or a polymer stabilised liquid crystal (PSLC) .

The method may comprise forming an electroluminescent (EL) layer such that the liquid crystal layer is mounted in front of the EL layer and printing the second electrode over the EL layer such that the first and second electrodes are arranged to generate, in use, an electric field across both the EL layer and the LC layer. Accordingly, the method may be used to form a Hybrid Display in which the LC layer and EL layer are driven by a common set of electrodes. The barrier layer may be formed between the LC layer and the EL layer to restrict migration of liquid crystal from the liquid crystal layer to the EL layer.

Accordingly, the display may comprise an electroluminescent (EL) layer, the layer of physically stabilised liquid crystal switchable to define the information to the displayed mounted in front of the EL layer, at least one pair of electrodes arranged to generate, in use, an electric field across both the EL layer and the LC layer.

The polymer matrix of the liquid crystal layer may be any one of water based, monomer free radiation curable urethane oligomer dispersions; acrylic functional polyurethane dispersions and acrylic urethane emulsions . In the most preferred embodiment, the polymer matrix is a UV curable polymer matrix, for example a UV curable aliphatic polyurethane resin, such as those supplied by DSM Neoresin under the trade name NeoRez®. However, it is believed that the matrix may comprise other film forming UV curable polymers, for example, UV curable polyurethane dispersions (known in the art as UV-PUD) , acrylic dispersions, silicones and mixtures therefore. The matrix resin can be formed from an aqueous solution or emulsion that contains very low levels, and preferably no, co-solvent.

Preferably, the polymer matrix comprises substantially no PVA. The term "substantially no PVA" , means the polymer matrix comprises less than 5% PVA, preferably less than 1% PVA and most preferably, no PVA.

A number of different component materials could be used to form a shell of the vesicles. One example of a pair of component materials that may be used for the shell is a multifunctional iscocyanate (e.g. the Desmodur range - sold by Bayer) and a diamine, such as ethylene diamine. The reaction may be catalysed by a tertiary amine, such as DABCO.

Notionally, the liquid crystal may be any one of the main types such material - such as nematic and cholesteric or chiral nematic - the requirement is , generally, for a liquid crystal based material that allows polariserless high contrast electro-optical shuttering operation between a field "on" state that is optically transmissive and a base field "off" state that us less transmissive than the "on" state.

Preferably, the liquid crystal contains a dye. In one arrangement, the liquid crystal includes up to 6% by weight of a dye, preferably a dichroic dye. In a preferred arrangement, levels of dye in the liquid crystal are 3- 5% by weight. The dye attaches to the liquid crystal molecules and acts to obscure light when no field is applied across the liquid crystal but when a field is applied, the dye molecules are aligned for allowing the transmission of light. As the dye is attached to the liquid crystal, the barrier layer limits the migration of both the liquid crystal and dye to layers behind the liquid crystal layer.

From the front to back a display may comprise: an electrically-insulating transparent front layer known as the substrate, usually made of glass or plastic, such as polyethylene terephthalate (PET) or polyethylene naphthalate (PEN) ; a first electrically-conductive film, for example, made from a material such as indium tin oxide (ITO) , forming one electrode - the front electrode - of the backlight; a polymer dispersed liquid crystal (LC) layer, the liquid crystal vesicles including a dye; a barrier layer of polyvinyl alcohol; a dielectric layer having a dielectric strength of more than 2V/μm; and disposed over the rear face of the electrically-insulating layer or

EL layer, an electrically conductive, transparent film forming a second electrode-conductive film forming a second electrode - the rear electrode

- of the backlight, the second electrode formed from a polymer mixture of a sulphonated polystyrene and a polythiophene based polymer.

The display of the invention may incorporate disposed over the entire rear face of the substrate a single (front) electrode, and disposed over the rear face of the reflective electrically-insulating layer a patterned (rear) electrode defining areas of the liquid crystal layer that can be selected to be switched "on" or "off" . However, it is possible as an alternative to pattern the front electrode and to have a single "whole-face" rear electrode. Moreover, it is possible for both electrodes to be patterned - as will need be the case if the display is going to be a matrix device where a multitude of very small areas can be illuminated at will so as to enable almost any shape and size of displayed image simply by selecting which areas are lit and which are dark.

The display may further comprise an electroluminescent (EL) layer (usually a particulate phosphor within a binder matrix) between the barrier layer and the dielectric layer.

According to an fifth aspect of the invention there is provided use of a dielectric layer in a display, the display comprising a physically stabilised liquid crystal layer between a first electrode and a printed, second electrode, the dielectric layer located between the physically stabilised liquid crystal layer and the second electrode, to prevent the ink used to form the second electrode from entering pin-holes in the physically stabilised liquid crystal layer during manufacture of the display. According to a sixth aspect of the invention there is provided a method of reducing fading of a display comprising a mask defining the information to be displayed, the mask comprising a layer of physically-stabilised liquid crystal comprising liquid crystal vesicles including a dye and a first electrode and a printed, transparent second electrode arranged to generate, in use, an electric field across the LC layer, the method comprising providing a barrier layer between the LC layer and the second electrode that restricts migration of liquid crystal together with the dye from the LC layer to layers rearward of the LC layer.

The method may comprise identifying a material that, when used as a barrier layer, limits the migration of liquid crystal from the LC layer to layers behind the LC layer such that no significant fading of the display occurs due to migration of the liquid crystal and using the identified material for the barrier layer.

According to a seventh aspect of the invention there is provided a method of determining a material suitable for use as a barrier layer in a display comprising :- forming a display comprising a mask defining the information to be displayed, the mask comprising a layer of physically-stabilised liquid crystal and a first electrode and a printed, transparent second electrode arranged to generate, in use, an electric field across the LC layer and a layer of material between the LC layer and the second electrode that has potential as the material of the barrier layer, heating the display to a predetermined temperature for a predetermined length of time, and examining the display for visible signs of liquid crystal migration to layers behind the LC layer. According to a eighth aspect of the invention there is provided a method of manufacturing a display comprising forming a first electrode on a substrate, forming a mask defining the information to be displayed, the mask comprising a layer of physically stabilised liquid, forming a barrier layer, forming a transparent second electrode such that a voltage can be applied to the first and second electrodes to generate an electric field across the LC layer, wherein the barrier layer is made of material identified as suitable for use as a barrier layer in accordance with the tenth aspect of the invention.

According to a ninth aspect of the invention there is provided a display comprising a mask defining the information to be displayed, the mask comprising a layer of physically-stabilised liquid crystal switchable to define the information to the displayed, a first electrode and a printed, transparent second electrode arranged to generate, in use, an electric field across the LC layer and a barrier layer between the mask and the second electrode, wherein the barrier layer is made of material identified as suitable for use as a barrier layer in accordance with the tenth aspect of the invention.

Embodiments of the invention will now be described, by example only, with reference to the accompanying drawings, in which :-

Figure 1 shows a section through a display according to an embodiment of the invention;

Figure 2 shows the display of Figure 1 in plan view;

Figure 3 is a table illustrating the suitability of different materials as a barrier layer; and Figure 4 shows a section through a display comprising an LC shutter and separate backlight.

The structure of the embodiment of the display of the invention depicted in Figure 1 of the accompanying drawings can be seen to be, from front to back: a relatively thick protective electrically-insulating transparent front layer (11 ; the substrate) ; over the rear face of the substrate 11 , a very thin transparent electrically-conductive film (12) forming a front (first) electrode of the display; covering the rear face of the front electrode 12, a relatively thin layer 13 of LC material 14 physically-stabilised by being dispersed within a supporting polymer matrix 15 (PDLC layer) ; formed directly on, and covering the rear face of the liquid crystal layer 13, a relatively thin barrier layer 10 of PVA that restricts migration of liquid crystal from the PDLC layer 13 to layers 16 , 19,20 to the rear of the LC layer; a relatively thin layer of electroluminescent/phosphor material 17 dispersed within a supporting matrix 18; over the rear face of the phosphor layer 16, a 5μm thick optically- transparent electrically-insulating dielectric layer 19 (in the Figure this layer is shown as a seamless extension of the phosphor layer 16; and disposed over the rear face of the electrically-insulating layer 19, a transparent electrically-conductive film 20 forming the rear (second) electrode(s) of the display, of PEDOT;PSS.

The front and rear electrodes together define discrete areas of both the liquid crystal layer and the electroluminescent layer that can be selected to be switched "on" or "off" . In this way, the LC layer defines a mask defining the information to be displayed and the EL layer providing a source of light to illuminate the areas defined by the mask.

In addition, the back electrode layer may be covered with a protective film (not shown here) . The dielectric constant of the electrically insulating layer is around 8 to 15 and the dielectric strength is around 20V/μm.

The PDLC layer 13 of the display is formed in the manner described in GB 0625114.4 with liquid crystal vesicles encapsulated in a UV cured polyurethane matrix. The liquid crystal vesicles include up to 6% by weight of a dye, preferably a dichroic dye, such that the PDLC layer 13 is switchable between a transmissive state and absorbing state (in which the PDLC layer 13 is predominately absorbing) .

It will be understood that the term "relatively thick" means thicknesses in the range of 30 to 300 micrometres. Furthermore, it will be understood that the term "relatively thin" means thicknesses of 50 micrometres or less. In a preferred embodiment, the relatively thick layers are around 100 micrometres and the relatively thin layers are 25 micrometres or less.

Figure 2 is an example of the types of information that may appear on the display.

The display may be manufactured substantially as is described in WO 2005/0121878, International patent application No: WO2008075001 and UK patent application No:0865751.5. However, the second electrode 16 may be formed by screen-printing PEDOT: PSS material onto the phosphor layer 19. After printing of the second electrode, an aqueous wash of the resultant film may be performed to remove any excess PSS from the film. In this embodiment, the aqueous wash comprises soaking of the film in water for an hour.

A number of materials were evaluated as barrier layers by coating a layer of diluted polymer solution onto a sample of PDLC prepared as described in GB 0625114.4. After drying the layer an EL lamp construction was printed onto the rear side of the layer of diluted polymer solution. Each display was then aged by placing the display in an oven held at 85⁰C for approximately 18 to 24 hrs, at which point it was examined for visible signs of liquid crystal/dye migration. A second display having a barrier layer of each material was characterised electro-optically before and after a similar aging process.

Figure 3 illustrates the results of these tests. As can be seen from Figure 3, out of the materials tested, PVA has been identified as particularly suitable for use as the barrier layer and a mixture of this polymer and an adhesion promoting polymer has also been identified as suitable depending on the required electro-optical performance of the display.

Further samples of test displays prepared using an alternative PVA as a barrier layer were prepared and subjected to a hot-humid aging test at 65°C/90%RH while being driven. It was found that the displays functioned after this test with a small degradation in performance. This was surprising, as it has been found previously that use of a hydrophilic polymer, such as PVA, as the polymer matrix of the PDLC layer can result in poor environmental stability.

It is envisaged that other materials, in particular, other hydrophilic polymers may be suitable for use as a barrier layer. It is believed these suitable materials can be determined by testing the material in the manner described above. The suitability of the material for the barrier layer will depend on the required performance for the display. Therefore, the temperature to which the display is heated and the time for which the display is heated during the test may be varied depending on the required performance. Materials identified as suitable for a barrier layer then can be used to manufacture a display in accordance with the invention.

It is expected that suitable materials will be those in which liquid crystal has low solubility.

In some barrier layers it may also be advantageous to include a polymer to improve the adhesion of the barrier layer with the LC layer and/or the EL layer.

Referring now to Figure 4, there is shown a display comprising an LC shutter 101 according to an embodiment of the invention. The structure of the embodiment of the LC shutter 101 from front to back comprises: a relatively thick protective electrically-insulating transparent front layer (111 ; the substrate) ; over the rear face of the substrate 111 , a very thin transparent electrically-conductive film 112, of for example ITO, forming the front (first) electrode of the display; covering the rear face of the front electrode 112, a relatively thin layer 113 of LC material 114 physically-stabilised by being dispersed within a supporting polymer matrix 115 (PDLC layer) ; formed directly on, and covering the rear face of the liquid crystal layer 113, a relatively thin barrier layer 110 of PVA that restricts migration of liquid crystal from the PDLC layer 113 to layers 116, 119, 120 to the rear of the LC layer; over the rear face of the barrier layer 110, an optically-transparent electrically-insulating dielectric layer 119, having a dielectric constant of around 8 to 15 and a dielectric strength around 20V/μm, such as polyvinylidene fluoride (PVDF) , a PVDF coploymer or a cyano-resin, and a thickness of 5μm or less; and disposed over the rear face of the reflective electrically-insulating dielectric layer 119, a transparent electrically-conductive film 120 forming the rear (second) electrode (s) of the display.

This embodiment of the display further comprises a backlight 121 rearward of the rear electrode 120 providing means to illuminate the LC shutter 101. The backlight 121 may be an electroluminescent, LED, incandescent or other suitable backlight. The backlight 121 may comprise a waveguide to direct light from the light source to illuminate the LC shutter 101.

It will be understood that in a different embodiment, the dielectric layer 119 may be a reflective dielectric material, such as a barium titanate loaded polymer resin, to form a reflective LC shutter and no backlight is required.

Like the PDLC layer 13, the PDLC layer 113 of the display is formed in the manner described in GB 0625114.4 with liquid crystal vesicles encapsulated in a UV cured polyurethane matrix. The liquid crystal vesicles include up to 6% by weight of a dye, preferably a dichroic dye, such that the PDLC layer 113 is switchable between a transmissive state and absorbing state (in which the PDLC layer 113 is predominately absorbing) .

The LC shutter 101 may be manufactured in a similar manner to the hybrid display described with reference to Figures 1 and 2. Again, the second rear electrode 120 may be formed by screen-printing PEDOT:PSS material onto the phosphor layer 119. The dielectric layer 119 blocks the ink used to form the second electrode 120 from the second rear electrode 120 from entering pin-holes in the LC layer 113. In this embodiment, the dielectric layer 119 is made of transparent materials rather than reflective materials as light from the backlight 121 has to pass through the LC shutter 101. After printing of the second electrode, an aqueous wash of the resultant film may be performed to remove any excess PSS from the film.

Claims

1. A method of manufacturing a display comprising providing a substrate having a first electrode formed thereon and coated with a physically stabilised liquid crystal layer and printing a second transparent electrode over the physically stabilised layer.

2. A display comprising a physically stabilised liquid crystal layer between a first electrode and a printed, transparent second electrode.

3. A method of claim 1 or the display of claim 2, wherein the second electrode is formed from a polymer mixture of a sulphonated polystyrene and a polythiophene based polymer.

4. A method or display of claim 3 , wherein the sulphonated polystyrene carries a negative charge.

5. A method or display of claim 4, wherein the negative charge is arrived at by deprotonation of part of the sulphonyl groups.

6. A method or display of any one of claims 3 to 5 , wherein the sulphonated polystyrene is sodium polystyrene sulphonate.

7. A method or display of any one of claims 3 to 6, wherein the polythiophene based polymer is a conjugated polymer.

8. A method or display of claim 7, wherein the polythiophene based polymer carries a positive charge.

9. A method or display of claim 8, wherein the polythiophene based polymer is a polyethylenedioxythiophene.

10. A method or display of claim 9, wherein the polyethylenedioxythiophene polymer is 3,4-ethylenedioxythiophene.

11. A method or display according to any one of claims 3 to 10, wherein the polymer mixture is a macromolecular salt where both components are charged.

12. A method or display according to any one of claims 3 to 11 , wherein the polymer mixture is PEDOT:PSS.

13. A method of manufacturing a display according any one of claims 1 and 3 to 12, wherein the display is an LC shutter and the method comprises forming a dielectric layer over the physically stabilised LC layer and printing the second electrode over the physically stabilised liquid crystal layer and the dielectric layer.

14. A display according to any one of claims 2 to 12, wherein the display comprises an LC shutter, the LC shutter comprising the physically stabilised liquid crystal layer between the first electrode and the second electrode and a dielectric layer between the physically stabilised liquid crystal layer and the second electrode.

15. A method according to claim 13 or a display according to claim 14, wherein the dielectric layer has a dielectric strength of more than 2V/μm,

16. A method or display according to claim 15 or a display, wherein the dielectric layer has a dielectric strength of around 20V/μm,

17. A method of any one of claims 13 and 14 to 16 or a display of any one of claims 14 to 16, wherein the dielectric layer is transparent.

18. A method of claim 17 comprising providing a backlight rearward of the second electrode, or the display of claim 17 comprising a backlight rearward of the second electrode.

19. A method of any one of claims 13 and 14 to 18 or display of any one of claims 14 to 18, wherein the dielectric layer includes polyvinylidene fluoride (PVDF) or a polyvinylidene fluoride coploymer.

20. A method of any one of claims 13 and 14 to 18 or display of any one of claims 14 to 18, wherein the dielectric layer includes a cyano- resin.

21. A method of any one of claims 13 to 16 or a display of any one of claims 14 to 16, wherein the dielectric layer is reflective.

22. A method of any one of claims 13 and 15 to 21 or the display of any one of claims 14 to 21 , wherein the dielectric layer is printed directly onto the physically stabilised liquid crystal layer.

23. A method of any one of claims 13 and 15 to 22 or the display of any one of claims 14 to 22, wherein the dielectric layer has a thickness of less than 25μm

24. A method or display of claim 23, wherein the dielectric layer has a thickness of less than 5μm.

25. A method of any one of claims 13 and 15 to 24 or a display of any one of claims 14 to 24, wherein the dielectric layer has a dielectric constant greater than 3.

26. A method or a display of claim 25, wherein the dielectric layer has a dielectric constant greater than 5.

27. A method or a display of claim 26, wherein the dielectric layer has a dielectric constant of 8 to 15.

28. A method of manufacturing a display according to any one of claims 1 , 3 to 13 and 15 to 27, comprising performing a wash of the second electrode to remove corrosive materials left over from printing of the second electrode.

29. A method of claim 28, wherein the wash is substantially an aqueous wash.

30. A method of any one of claims 1 , 3 to 13 and 15 to 29 comprising forming a barrier layer that restricts migration of liquid crystal from the physically stabilised liquid crystal layer between the physically stabilised layer and the second electrode.

31. A display of any one of claims 2 to 12 and 14 to 27, comprising a barrier layer between the physically stabilised liquid crystal layer and the second electrode, the barrier layer restricting migration of liquid crystal from the physically stabilised liquid crystal layer.

32. A method according to claim 30 or a display according to claim 31 , comprising an LC shutter, the LC shutter comprising the physically stabilised liquid crystal layer, the first electrode, the second electrode and the barrier layer.

33. A method according to claim 32, comprising providing a backlight rearward of the second electrode for illuminating the LC shutter or a display according to claim 32, comprising a backlight rearward of the second electrode for illuminating the LC shutter.

34. A method according to claim 31 or claim 33 or a display according to claim 32 or claim 33, not comprising an EL layer that is driven by the first and second electrodes .

35. A method according to claim 31 comprising forming an EL layer behind the LC layer, between the barrier layer and the second electrode, or a display according to claim 32, comprising an EL layer behind the LC layer, between the barrier layer and the second electrode.

36. A method according to any one of claims 31 and 33 to 35 or a display according to any one of claims 32 to 35, wherein the barrier layer is formed directly on the physically stabilised liquid crystal layer.

37. A method according to any one of claims 30 and 32 to 36 or a display according to any one of claims 31 to 36, wherein liquid crystal vesicles of the physically stabilised liquid crystal layer include a dye and the barrier layer is arranged to limit the migration of liquid crystal together with the dye from the LC layer to layers behind the LC layer such that no significant fading of the display occurs due to migration of the liquid crystal together with the dye under pre-determined criteria.

38. A method according to any one of claims 30 and 32 to 37 or a display according to any one of claims 31 to 37, wherein the barrier layer is arranged to limit the migration of liquid crystal from the LC layer to layers behind the LC layer such that, when the display is heated to 85 degrees over 18hrs, there is virtually no change in contrast between illuminated and non-illuminated areas of the display.

39. A method according to any one of claims 30 and 32 to 38 or a display according to any one of claims 31 to 38, wherein the barrier layer comprises a material in which the liquid crystal has low solubility.

40. A method or display according to claim 39, wherein the barrier layer comprises a material in which the liquid crystal has substantially zero solubility at room temperature and atmospheric pressure.

41. A method according to any one of claims 30 and 32 to 40 or a display according to any one of claims 31 to 40, wherein the barrier layer comprises a hydrophilic layer.

42. A method or display according to claim 41 , wherein the barrier layer is a hydrophilic polymer.

43. A method or display according to claim 42, wherein the polymer is a water soluble polymer.

44. A method or display according to claim 43, wherein the water soluble polymer is one of a polyvinyl alcohol, polyvinyl butyral, polyethylene glycol, polypropylene glycol, gelatine and its derivatives, cellulose derivatives polyacrylics and derived polymers and polyacrylic acids and derived polymers.

45. A method or display according to claim 42, wherein the polymer is a non-water soluble polymer that can be carried in an emulsion and/or dispersion.

46. A method or display of claim 45, wherein the non-water soluble polymer is one of a polyurethanes, polyethylene-acylic acid copolymer and derived copolymers, polymethacrylates and derived copolymers and polymethacrylic acids and derived copolymers.

47. A method according to any one of claims 30 and 32 to 44 or a display according to any one of claims 31 to 44, wherein the barrier layer comprises a polyvinyl alcohol (PVA) .

48. A method according to any one of claims 31 and 33 to 47 or a display according to any one of claims 32 to 47, wherein the barrier layer comprises a polymer in which the liquid crystal has low solubility and an adhesion promoter to improve the adhesion of the barrier layer with the LC layer.

49. Use of a dielectric layer in a display, the display comprising a physically stabilised liquid crystal layer between a first electrode and a printed, transparent second electrode, the dielectric layer located between the physically stabilised liquid crystal layer and the second electrode, to prevent the ink used to form the second electrode from entering pin-holes in the physically stabilised liquid crystal layer during manufacture of the display,

50. A method of reducing fading of a display comprising a mask defining the information to be displayed, the mask comprising a layer of physically-stabilised liquid crystal comprising liquid crystal vesicles including a dye and a first electrode and a printed, transparent second electrode arranged to generate, in use, an electric field across the LC layer, the method comprising providing a barrier layer between the LC layer and the second electrode that restricts migration of liquid crystal together with the dye from the LC layer to layers rearward of the LC layer.

51. A method of determining a material suitable for use as a barrier layer in a display comprising :- forming a display comprising a mask defining the information to be displayed, the mask comprising a layer of physically-stabilised liquid crystal and a first electrode and a printed, transparent second electrode arranged to generate, in use, an electric field across the LC layer and a layer of material between the LC layer and the second electrode that has potential as the material of the barrier layer, heating the display to a predetermined temperature for a predetermined length of time, and examining the display for visible signs of liquid crystal migration to layers behind the LC layer.

52. A method of manufacturing a display comprising forming a first electrode on a substrate, forming a mask defining the information to be displayed, the mask comprising a layer of physically stabilised liquid, forming a barrier layer, printing a transparent second electrode such that a voltage can be applied to the first and second electrodes to generate an electric field across the LC layer, wherein the barrier layer is made of material identified as suitable for use as a barrier layer in accordance with claim 51.

53. A display comprising a mask defining the information to be displayed, the mask comprising a layer of physically-stabilised liquid crystal switchable to define the information to the displayed, a first electrode and a printed, transparent second electrode arranged to generate, in use, an electric field across the LC layer and a barrier layer between the mask and the second electrode, wherein the barrier layer is made of material identified as suitable for use as a barrier layer in accordance with claim 51.

54. An electroluminescent display substantially as described herein with reference Figures 1 and 2.

55. A LC shutter substantially as described with reference to Figure 4.