WO1997040416A1

WO1997040416A1 - Liquid crystal display cell using phosphor emitters

Info

Publication number: WO1997040416A1
Application number: PCT/GB1997/001091
Authority: WO
Inventors: William Alden Crossland; Anthony Bernard Davey; Vincent Glenn Geake; Ian David Springle; Paul Andrew Bayley
Original assignee: The Secretary Of State For Defence
Priority date: 1996-04-19
Filing date: 1997-04-18
Publication date: 1997-10-30
Also published as: AU2394797A; GB9608090D0; EP0894283A1; JP2000508788A

Abstract

A liquid crystal display device comprises a modulatable liquid-crystal layer (1) between transparent substrates (2, 3) in the usual way for modulating light (12) at a first (preferably UV) wavelength so that according to the voltage applied via electrodes (4, 5) it selectively reaches an array of phosphor emitters (10) in between the liquid crystal layer and the upper substrate (3). The phosphors (or other photoluminescent material) emit or reflect light at visible wavelengths when excited by light at the first wavelength. The significant feature is that the phosphors (10) are located inside the cell, which means that no special collimating measures are needed, and this is achieved by configuring the phosphors to have a surface of a smoothness comparable to that of the glass, for instance by suspending them in a spun polymer layer, or by applying a planarisation layer, or by using fluorescent glass.

Description

- i -

LIOUID CRYSTAL DISPLAY CELL USING PHOSPHOR EMITTERS

Recent designs of liquid crystal displays have used modulation of ultraviolet light by liquid crystal cells, the ultraviolet light being used to cause red, green and blue phosphors to radiate in order to produce a colour display; this dispenses with the need for colour filters. In many such designs, such as GB 2274191, the phosphors are underneath the liquid crystal and the liquid crystal therefore modulates the visible light emitted by the phosphors. This means that the display suffers from the usual problems of directionality of viewing.

A more promising approach is that used for instance in the international application No.WO95/27920 (Crossland et al) , in which the phosphor dots are placed on top of the liquid crystal cell, so that the liquid crystal layer modulates ultraviolet light and this modified ultraviolet light then hits the phosphor dots, which are viewed more or less directly by the viewer, as with a CRT screen. This kind of display is clear and easy to read at almost any angle. A problem with it is that the thickness of the glass encapsulating the liquid crystal layer is quite large compared to the spacing of the pixels. This means that it is difficult to ensure that the ultraviolet light modulated by an element of the liquid crystal layer is accurately directed onto the corresponding phosphor dot. One approach to solving this problem is to use collimation, as in WO95/27920. The present invention, however, is concerned with an alternative approach in which the phosphor dots are located inside the liquid crystal construction, that is to say in between the glass plates and adjacent to the liquid crystal itself. Such a construction is shown in US-4 830 469 (US Philips Corporation) . Here the aim is to use an STN liquid crystal to produce a colour display for television. The usual problem of untrue colour display caused by the dispersion of the optical properties is overcome by dispensing with at least one polariser and using ultraviolet light to activate phosphors as described above. Since because of the presence of the phosphor inside the cell there is no analyser the ultraviolet light must be capable of being stopped by the liquid crystal layer itself when suitably addressed, and this is achieved by using a guest-host system, that is to say a liquid crystal "host" containing a "guest" in the form of a dichroic dye. This dye, which may itself have a similar composition to the liquid crystal, absorbs the light in the LC cell when the cell is in the twisted state, i.e. with no voltage applied. In this state the UV light is prevented from reaching the phosphor and the cell is dark. Although the patent discusses at some length the liquid crystals and dyes to be used, there is no information on the critical question of how the phosphors are to be incorporated into the cell.

Prior art phosphors are typically doped semiconductors. The semiconductor is chosen to have a band gap slightly larger than an excited state of the dopant, so that energy can be easily transferred to the dopant that will then relax with emission of a photon. The nature of the dopant and its concentration therefore determine the emitted light. Typical combinations are zinc sulphide:silver (blue) , zinc sulphide:copper (green) and yttrium oxysulphide: europium (red) , and indeed these are the phosphors mentioned in the aforementioned patent US 4830469 (Philips) . These phosphors are normally deposited by heating mixtures of the materials to about 1200°C, using additional fluxes of alkali and alkaline earth halides and oxides to provide a low temperature melting phase. This produces a mass with the grain size typically 5-15μm. Liquid crystal cells are often very thin, with a thickness sometimes down to 1.5/ιm though more typically about 5μm in the case of STN. As can be seen, this is of the same order of magnitude as the particle size of standard phosphors. Such standard phosphors cannot therefore be incorporated into the liquid crystal cell, since the unevenness in size would ruin the orientation of the liquid crystals and the uniformity of the electric field. The phosphors cannot be ground smaller without loss of efficiency because grinding processes introduce too many defects into the grain structure. In US 4830469 (Philips, supra) such standard phosphors are said to be used inside a UVLCD cell . However, the patent is silent about the method of deposition and methods to reduce particle size. It is not therefore clear from this disclosure how such a cell can be made to work, particularly since STN liquid crystals need two polarisers to modulate light and cannot be used with dichroic dyes.

According to one aspect of the invention there is provided a liquid crystal display device comprising a modulatable liquid-crystal layer adjacent to a transparent substrate, the liquid crystal layer being adapted to modulate light at a first wavelength, and further including an array of secondary emitters in between the liquid crystal layer and the substrate, for emitting or reflecting light at a second wavelength when excited by light at the first wavelength, characterised in that the array of secondary emitters is formed from a material with a surface roughness having peak-to-trough variations of less than about 5% of the cell thickness, in typical applications O.lμm, over a distance of the order of lOμm. This requirement is of the same order as that of the glass surface itself. The surface must also be flat, i.e. with little variation from pixel to pixel. The secondary emitter layer in one embodiment contains phosphor material having a particle size less than O.lμm, embedded in a smooth material such as a polymer or gel. The polymer surface then gives the required smoothness . The polymer layer can then be deposited on the electrode layer used to address the liquid crystal, or it can be part of it, and an alignment layer can then be deposited on the polymer layer. Alternatively, if the polymer layer is of a suitable material, it could be rubbed and itself constitute the alignment layer.

The phosphors can be such as are described in Yu et al . Optical properties of homogeneously Mn-doped ZnS nano-particles' / presented to the First International Conference on the Science and Technology of Display Phosphors, 14-16 November 1995, San Diego. Such inorganic phosphors have a size of about 20nm and can be made into layers having the smoothness required for the invention.

If owing to clumping of phosphor particles or for any other reason the surface of the photoluminescent layer is still not smooth enough then in a modification of the above-described system supplementary planarisation step is implemented. A possible technique for this planarisation is to utilise a resin, in particular a resin such as Benzocyclobutene (BCB) , as offered under the trade name 'Cyclotene' by Dow Chemical Company and as used for different planarisation purposes in other types of liquid crystal display (see for example D.J. Perettie, M. McCulloch and P.E. Garrou, 'Benzocyclobutene as a planarisation resin for flat panel displays' , in Proceedings of the SPIE: Liquid Crystal Materials, Devices and applications, Vol. 1665, pages 331-337; SPIE/IS&T, 1992) . This material may be spin-coated onto the surface of the phosphor matrix layer and has good self- planarising properties. A conductive coating may subsequently be applied as the next step in the production of a cell.

The planarising effect of the additional layer means, moreover, that using a strongly self-planarising resin such as BCB, larger-particle phosphors can be incorporated into such a display.

Examples of the formation of the phosphor layer are as follows.

In a first method standard commercially available phosphors, which are typically 1 - 10 micron in particle size, are screen-printed or electrophoretically deposited onto the substrate and patterned in an appropriate way. This is followed by planarisation using a resin such as BCB, which can smooth out irregularities up to, say, l-2μ, and then a conductive coating would be deposited in the next step of cell manufacture. As before the surface of the phosphor layer/planarisation layer composite will have the required smoothness, while allowing the use of standard, and hence cheap, phosphor powders.

An alternative approach again would be to mix the phosphor into a suitable amount of BCB, deposit the layer and then pattern the BCB/phosphor directly using photolithographic steps. This is possible because certain forms of BCB can themselves be optically polymerised in the same way as negative photoresist.

Other methods which could be used for the planarisation of the phosphor layers are : - * To use a transparent lacquer film formed from a drop on the surface of the water in a bath containing the phosphor layer; the film is then dropped onto the surface of the phosphor layer as the water is drained from the bath. This would be followed by a conductive coating, as required.

* Polymer casting:- To cast a transparent polymer film over the phosphor layer using a flat hot plate. This would result in a flat surface which could be coated with a conductive material.

* Fluorescent glass could be used in place of a phosphor coated screen. This would be inherently flat and probably not require any significant planarisation.

* Thin film (sputtered/evaporated) phosphor layers could be utilised in place of powder phosphors. The surfaces and edges of the resulting phosphors (including the geometry of the pixel-surrounding matrix) could be treated to increase the forward emission of visible light from the films. In a still further modification the secondary emitters comprise organic fluorescent materials; these may be standard fluorescent materials, such as fluorescein. Alternatively, the emitters may comprise doped organic materials such as polystyrene-poly(3- hexylthiophene) (PS-P3HT) diblock copolymer.

By the use of organic phosphors it is possible to construct a workable cell because the organic materials can be deposited by thin film processes which do not interfere either mechanically or in terms of sheer bulk with the liquid crystal layer and its electrodes.

In some embodiments of the invention, to enhance the contrast of the display, a thin dielectric film that transmits light of the first frequency and reflects light at the second frequency may be provided between the liquid crystal layer and the secondary emitters . The liquid crystal layer may comprise dichroic dyes in short-pitch chiral nematic liquid crystals which modulate ultraviolet light directly under an applied electric field. The liquid crystal layer may alternatively comprise dichroic dyes in nematic or (high-tilt) ferroelectric liquid crystals including short-pitch SmC* devices, in which case the liquid crystal display must also comprise a polariser. Some liquid crystals and e-o effects are more sensitive to cell thickness than others, so the planarity requirements of the secondary emitter layer vary according to the liquid crystal used.

One advantage of using dichroic dyes is that they can be operated without parallax and without collimation because either no polarisers or only one are needed, as explained in the Philips patent, and hence the phosphor can be incorporated inside the cell. Another way that phosphors can be incorporated into a cell is with cholesteric mirrors and a nematic- cholesteric phase change switch, but these systems need collimation because the cholesteric mirror effect depends on the angle of incidence.

The substrate will usually have transparent electrodes for addressing the LC. If these are located underneath the phosphor layer (i.e. nearer the substrate) then it is also desirable for the phosphor layer to be thin, say less than 0.5μm, to minimise voltage drop across it. Alternatively the electrodes can be deposited on top of the phosphor layer. This would be the normal arrangement and is feasible provided that the phosphor material can withstand the processing temperature for the ITO electrodes, i.e. about 250°C. However, even the organic phosphor materials contemplated fulfil this criterion. The small distance from liquid crystal to phosphor means that a collimator is not needed from the point of view of crosstalk, unlike the prior art structures of PCT/GB95/00770. The first wavelength may be in the ultra-violet (UV) , for example in the UVA region, or low wavelength visible light, which matches the characteristics of many common phosphors. Advantageously the liquid crystal display further comprises a source of monochromatic light. This can be a backlight or a system using edge lighting of a rear optical element with means to direct that light through the display, as is done in conventional liquid crystal displays used in computers. The source may be collimated or uncollimated.

According to another aspect of the invention therefore there is provided a method of manufacturing a liquid-crystal cell, including the step of depositing a photoluminescent material on a transparent substrate, possibly with a planarisation layer, to form a thin film having a roughness of less than 0. lμm over a distance of the order of lOμm and forming a liquid crystal layer on the photoluminescent material. The method of thin film deposition may be electrophoretic deposition of phosphor particles, as is known. Alternatively evaporation, RF sputtering, pulsed laser deposition or aerosol spray pyrolysis can be used, followed by high temperature annealing (above 1000°C) . The last-named method achieves flatness to perhaps lμm.

A further alternative method of deposition that may be used is combustion synthesis, as described by Kingsley and Patel (Materials letters 6 pages 427-432 (1988) ) . An oxidiser such as a metal nitrate may be reacted with an organic fuel such as urea (CH₄N₂0) , carbohydrazide (CH₆N₄0) or glycine (C₂H₅N0₂) in a bomb that also contains phosphor precursors such as zinc and sulphur with small amounts of dopants. This produces fine particles of phosphor.

A still further method of deposition is the use of MBE or MOCVD deposition of the phosphor, without subsequent annealing.

Preferably however nanocrystalline particles can be prepared which can then be incorporated into a liquid carrier, for instance a dissolved polymer or liquid monomer; this is then spread on the substrate to the required thickness and evaporated or polymerised to give a solid layer, which is then patterned as required. For RGB displays this process is carried out three times, one for each colour, following which an alignment layer may be deposited if the polymer layer itself is not used as an alignment layer.

For a better understanding of the invention embodiments will now be described with reference to the attached Figure which shows a cross-section through a liquid-crystal display system embodying the invention.

In the Figure, the liquid crystal layer, which may be for example a short-pitch chiral nematic liquid crystal, is sandwiched between two glass substrates 2, 3. The liquid crystal layer is modulatable in the usual way by orthogonally extending sets of electrode strips 4 and 5 of a transparent conductor such as indium tin oxide.

Ultraviolet light 12 is directed at the underside of the first glass plate 2 and passes through the liquid-crystal layer at points where a voltage is applied between the electrodes 4 and 5, and is absorbed by the liquid crystal layer where no voltage is applied. At the points where the ultraviolet light passes through the liquid crystal it traverses the second electrode layer 5 and impinges upon phosphor dots 10, causing them to emit.

In order to manufacture a cell, first a glass substrate is provided. A thin film of polymer containing one colour phosphor is deposited, on which photoresist is deposited and patterned. The assembly is then etched, leaving the phosphor only where required. These steps are repeated for the other two colours. The display is then completed by depositing indium tin oxide (ITO) layers and completing the manufacture in the usual way. If the electrical properties of the phosphor layer allow it the ITO electrodes can be underneath the phosphor layer, though this would not normally be the preferred configuration. Because the phosphor layer 10 is smooth it does not interfere with the liquid crystal layer 1, which may be of the order of 5 μm thick. Moreover the phosphors in question are particularly suited to use with near ultraviolet light of wavelength about 365nm. This means that in a variant of the invention the phosphors may actually be used on the outside of the glass substrate 3 even though in this configuration some collimation will need to be used.

In an alternative embodiment, instead of a phosphor a light-emitting conjugated polymer is used, such as is available from Cambridge Display Technology Ltd, GB, under the general designation Poly(p-xylene- alpha-tetrahydrothiophenium bromide) .

Although the embodiments described use phosphors the "emitters" referred to could be photochromic, changing their reflective or absorptive properties in response to activation light .

Claims

CLAIMS :

1. A liquid crystal display device comprising a modulatable liquid-crystal layer (1) adjacent to a transparent substrate (3) , the liquid crystal layer being adapted to modulate light at a first wavelength, and further including an array of secondary emitters (10) in between the liquid crystal layer and the substrate, for emitting or reflecting light at a second wavelength when excited by light at the first wavelength, characterised in that the array of secondary emitters is formed as a layer with a surface roughness on the liquid-crystal side having peak-to- trough variations of less than about 5% of the cell thickness, in particular less than about 0.1 μm, over a distance of the order of 10μm.

2. A display device according to claim 1, in which the secondary^"emitter layer (10) is formed of phosphor-type particles embedded in an organic material.

3. A display device according to claim 2, in which the organic material functions as an alignment layer for the liquid crystal .

4. A display device according to claim 1 or 2 , in which the secondary emitter layer (10) includes a planarisation layer forming the said surface.

5. A display device according to claim 1, in which the secondary emitter layer (10) is formed of a fluorescent glass.

6. A display device according to claim 1, in which the secondary emitter layer is made of an organic fluorescent material, such as fluorescein, or of a doped organic semiconductor, such as polystyrene- poly(3-hexylthiophene) (PS-P3HT) diblock copolymer.

7. A display device according to any preceding claim, in which a thin dielectric film that transmits light of the first frequency and reflects light at the second frequency is provided between the liquid crystal layer and the secondary emitters .

8. A display device according to any preceding claim, in which the liquid crystal layer comprises a dichroic dye in a short-pitch chiral nematic liquid crystal or in a nematic or (high-tilt) ferroelectric liquid crystal.

9. A display device according to any preceding claim and including a transparent electrode for addressing the liquid crystal, located on the liquid- crystal side of the secondary emitters.

10. A liquid crystal display device comprising a modulatable liquid-crystal layer (1) adjacent to a transparent substrate (3) , the liquid crystal layer being adapted to modulate light at a first wavelength, and further including an array of secondary emitters (10) in between the liquid crystal layer and the substrate, for emitting or reflecting light at a second wavelength when excited by light at the first wavelength, characterised in that the array of secondary emitters is formed as a layer with a surface on the liquid-crystal side having a planarity comparable to that of standard liquid crystal glasses for the liquid crystal used.

11. A method of manufacturing a liquid-crystal cell, including the step of forming a secondary emission layer (10) on a transparent substrate (3) as a thin film having a roughness of less than about 0. lμm over a distance of the order of lOμm, and forming a liquid crystal layer (1) on the secondary emission layer.

12. A method according to claim 11, in which the formation of the secondary emission layer includes the step of depositing phosphors by screen printing, electrophoresis, evaporation, RF sputtering or pulsed laser deposition, followed by high temperature annealing, or by combustion synthesis, or by MBE or MOCVD without subsequent annealing, onto the substrate.

13. A method according to claim 11 or 12, in which the formation of the phosphors is followed by planarisation using an organic material.

14. A method according to claim 13, in which the phosphor particle size is l-10μm.

15. A method according to claim 11 or 13, in which the secondary emission layer or, as the case may be, the planarisation layer is formed by spinning or casting.

16. A method according to claim 11, in which the secondary emission layer is made by incorporating nanocrystalline particles of phosphor material into a liquid carrier, or by preparing a dissolved polymer or liquid monomer of photoluminescent material, and the liquid is then spread on the substrate to the required thickness and evaporated or polymerised to give a solid layer, which is then patterned as required.