EP0894283A1 - Liquid crystal display cell using phosphor emitters - Google Patents
Liquid crystal display cell using phosphor emittersInfo
- Publication number
- EP0894283A1 EP0894283A1 EP97919502A EP97919502A EP0894283A1 EP 0894283 A1 EP0894283 A1 EP 0894283A1 EP 97919502 A EP97919502 A EP 97919502A EP 97919502 A EP97919502 A EP 97919502A EP 0894283 A1 EP0894283 A1 EP 0894283A1
- Authority
- EP
- European Patent Office
- Prior art keywords
- layer
- liquid crystal
- liquid
- display device
- light
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Withdrawn
Links
- 239000004973 liquid crystal related substance Substances 0.000 title claims abstract description 68
- OAICVXFJPJFONN-UHFFFAOYSA-N Phosphorus Chemical compound [P] OAICVXFJPJFONN-UHFFFAOYSA-N 0.000 title claims abstract description 43
- 239000000758 substrate Substances 0.000 claims abstract description 19
- 239000000463 material Substances 0.000 claims abstract description 18
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- 229920000642 polymer Polymers 0.000 claims abstract description 11
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- 239000002245 particle Substances 0.000 claims description 9
- -1 poly(3-hexylthiophene) Polymers 0.000 claims description 8
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- 239000010409 thin film Substances 0.000 claims description 5
- 238000000137 annealing Methods 0.000 claims description 4
- 239000004986 Cholesteric liquid crystals (ChLC) Substances 0.000 claims description 3
- 230000015572 biosynthetic process Effects 0.000 claims description 3
- 239000002105 nanoparticle Substances 0.000 claims description 3
- 239000004065 semiconductor Substances 0.000 claims description 3
- 238000005266 casting Methods 0.000 claims description 2
- 238000005049 combustion synthesis Methods 0.000 claims description 2
- 229920000359 diblock copolymer Polymers 0.000 claims description 2
- 230000008020 evaporation Effects 0.000 claims description 2
- 238000001704 evaporation Methods 0.000 claims description 2
- 239000005262 ferroelectric liquid crystals (FLCs) Substances 0.000 claims description 2
- GNBHRKFJIUUOQI-UHFFFAOYSA-N fluorescein Chemical compound O1C(=O)C2=CC=CC=C2C21C1=CC=C(O)C=C1OC1=CC(O)=CC=C21 GNBHRKFJIUUOQI-UHFFFAOYSA-N 0.000 claims description 2
- 238000002488 metal-organic chemical vapour deposition Methods 0.000 claims description 2
- 239000000178 monomer Substances 0.000 claims description 2
- 229920000301 poly(3-hexylthiophene-2,5-diyl) polymer Polymers 0.000 claims description 2
- 238000004549 pulsed laser deposition Methods 0.000 claims description 2
- 238000001552 radio frequency sputter deposition Methods 0.000 claims description 2
- 239000007787 solid Substances 0.000 claims description 2
- 230000003746 surface roughness Effects 0.000 claims description 2
- 238000001962 electrophoresis Methods 0.000 claims 1
- 238000007650 screen-printing Methods 0.000 claims 1
- 238000009987 spinning Methods 0.000 claims 1
- 210000004027 cell Anatomy 0.000 description 17
- UMIVXZPTRXBADB-UHFFFAOYSA-N benzocyclobutene Chemical compound C1=CC=C2CCC2=C1 UMIVXZPTRXBADB-UHFFFAOYSA-N 0.000 description 8
- 239000000975 dye Substances 0.000 description 7
- 239000011347 resin Substances 0.000 description 5
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- 210000002858 crystal cell Anatomy 0.000 description 4
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- 230000003098 cholesteric effect Effects 0.000 description 3
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- 230000000694 effects Effects 0.000 description 3
- 230000003287 optical effect Effects 0.000 description 3
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- CFAKWWQIUFSQFU-UHFFFAOYSA-N 2-hydroxy-3-methylcyclopent-2-en-1-one Chemical compound CC1=C(O)C(=O)CC1 CFAKWWQIUFSQFU-UHFFFAOYSA-N 0.000 description 2
- DHMQDGOQFOQNFH-UHFFFAOYSA-N Glycine Chemical compound NCC(O)=O DHMQDGOQFOQNFH-UHFFFAOYSA-N 0.000 description 2
- 239000005083 Zinc sulfide Substances 0.000 description 2
- 239000004020 conductor Substances 0.000 description 2
- 238000010276 construction Methods 0.000 description 2
- 230000005684 electric field Effects 0.000 description 2
- 238000005516 engineering process Methods 0.000 description 2
- AMGQUBHHOARCQH-UHFFFAOYSA-N indium;oxotin Chemical compound [In].[Sn]=O AMGQUBHHOARCQH-UHFFFAOYSA-N 0.000 description 2
- 239000011159 matrix material Substances 0.000 description 2
- 239000000203 mixture Substances 0.000 description 2
- 230000004048 modification Effects 0.000 description 2
- 238000012986 modification Methods 0.000 description 2
- 229920002120 photoresistant polymer Polymers 0.000 description 2
- 239000000843 powder Substances 0.000 description 2
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 2
- DRDVZXDWVBGGMH-UHFFFAOYSA-N zinc;sulfide Chemical compound [S-2].[Zn+2] DRDVZXDWVBGGMH-UHFFFAOYSA-N 0.000 description 2
- 239000001837 2-hydroxy-3-methylcyclopent-2-en-1-one Substances 0.000 description 1
- RYGMFSIKBFXOCR-UHFFFAOYSA-N Copper Chemical compound [Cu] RYGMFSIKBFXOCR-UHFFFAOYSA-N 0.000 description 1
- 229910052693 Europium Inorganic materials 0.000 description 1
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- NINIDFKCEFEMDL-UHFFFAOYSA-N Sulfur Chemical compound [S] NINIDFKCEFEMDL-UHFFFAOYSA-N 0.000 description 1
- 239000005864 Sulphur Substances 0.000 description 1
- XSQUKJJJFZCRTK-UHFFFAOYSA-N Urea Chemical compound NC(N)=O XSQUKJJJFZCRTK-UHFFFAOYSA-N 0.000 description 1
- HCHKCACWOHOZIP-UHFFFAOYSA-N Zinc Chemical compound [Zn] HCHKCACWOHOZIP-UHFFFAOYSA-N 0.000 description 1
- SKQWEERDYRHPFP-UHFFFAOYSA-N [Y].S=O Chemical compound [Y].S=O SKQWEERDYRHPFP-UHFFFAOYSA-N 0.000 description 1
- 230000004913 activation Effects 0.000 description 1
- 239000000443 aerosol Substances 0.000 description 1
- 239000003513 alkali Substances 0.000 description 1
- 230000008901 benefit Effects 0.000 description 1
- 239000004202 carbamide Substances 0.000 description 1
- XEVRDFDBXJMZFG-UHFFFAOYSA-N carbonyl dihydrazine Chemical compound NNC(=O)NN XEVRDFDBXJMZFG-UHFFFAOYSA-N 0.000 description 1
- 230000008859 change Effects 0.000 description 1
- 239000003086 colorant Substances 0.000 description 1
- 239000002131 composite material Substances 0.000 description 1
- 229920000547 conjugated polymer Polymers 0.000 description 1
- 229910052802 copper Inorganic materials 0.000 description 1
- 239000010949 copper Substances 0.000 description 1
- 239000013078 crystal Substances 0.000 description 1
- 230000007547 defect Effects 0.000 description 1
- 239000006185 dispersion Substances 0.000 description 1
- 238000001652 electrophoretic deposition Methods 0.000 description 1
- OGPBJKLSAFTDLK-UHFFFAOYSA-N europium atom Chemical compound [Eu] OGPBJKLSAFTDLK-UHFFFAOYSA-N 0.000 description 1
- 230000005281 excited state Effects 0.000 description 1
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- 230000008018 melting Effects 0.000 description 1
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- 229910052709 silver Inorganic materials 0.000 description 1
- 239000004332 silver Substances 0.000 description 1
- 238000005118 spray pyrolysis Methods 0.000 description 1
- 239000000126 substance Substances 0.000 description 1
- 238000000427 thin-film deposition Methods 0.000 description 1
- 229910052725 zinc Inorganic materials 0.000 description 1
- 239000011701 zinc Substances 0.000 description 1
Classifications
-
- G—PHYSICS
- G02—OPTICS
- G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
- G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour
- G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour based on liquid crystals, e.g. single liquid crystal display cells
- G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
- G02F1/1333—Constructional arrangements; Manufacturing methods
- G02F1/1335—Structural association of cells with optical devices, e.g. polarisers or reflectors
- G02F1/1336—Illuminating devices
- G02F1/133617—Illumination with ultraviolet light; Luminescent elements or materials associated to the cell
-
- G—PHYSICS
- G02—OPTICS
- G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
- G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour
- G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour based on liquid crystals, e.g. single liquid crystal display cells
- G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
- G02F1/1333—Constructional arrangements; Manufacturing methods
- G02F1/1335—Structural association of cells with optical devices, e.g. polarisers or reflectors
- G02F1/13356—Structural association of cells with optical devices, e.g. polarisers or reflectors characterised by the placement of the optical elements
- G02F1/133565—Structural association of cells with optical devices, e.g. polarisers or reflectors characterised by the placement of the optical elements inside the LC elements, i.e. between the cell substrates
Definitions
- 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.
- the ultraviolet light 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.
- 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 5-15 ⁇ m. Liquid crystal cells are often very thin, with a thickness sometimes down to 1.5/ ⁇ m though more typically about 5 ⁇ m 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.
- 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.l ⁇ m, over a distance of the order of lO ⁇ m. This requirement is of the same order as that of the glass surface itself. The surface must also be flat, i.e.
- the secondary emitter layer in one embodiment contains phosphor material having a particle size less than O.l ⁇ m, embedded in a smooth material such as a polymer or gel.
- 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.
- 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.
- BCB Benzocyclobutene
- 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.
- 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.
- Fluorescent glass could be used in place of a phosphor coated screen. This would be inherently flat and probably not require any significant planarisation.
- the secondary emitters comprise organic fluorescent materials; these may be standard fluorescent materials, such as fluorescein.
- the emitters may comprise doped organic materials such as polystyrene-poly(3- hexylthiophene) (PS-P3HT) diblock copolymer.
- organic phosphors 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.
- 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.
- dichroic dyes 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 0.5 ⁇ m, 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.
- 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.
- 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. l ⁇ m over a distance of the order of lO ⁇ m 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.
- 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 l ⁇ m.
- 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 (CH 4 N 2 0) , carbohydrazide (CH 6 N 4 0) or glycine (C 2 H 5 N0 2 ) 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.
- 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.
- a liquid carrier for instance a dissolved polymer or liquid monomer
- 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.
- 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.
- 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 ⁇ m thick.
- 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.
- a light-emitting conjugated polymer such as is available from Cambridge Display Technology Ltd, GB, under the general designation Poly(p-xylene- alpha-tetrahydrothiophenium bromide) .
- the embodiments described use phosphors the "emitters” referred to could be photochromic, changing their reflective or absorptive properties in response to activation light .
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- Physics & Mathematics (AREA)
- Nonlinear Science (AREA)
- Mathematical Physics (AREA)
- Chemical & Material Sciences (AREA)
- Crystallography & Structural Chemistry (AREA)
- General Physics & Mathematics (AREA)
- Optics & Photonics (AREA)
- Devices For Indicating Variable Information By Combining Individual Elements (AREA)
- Liquid Crystal (AREA)
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
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.
Applications Claiming Priority (3)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
GBGB9608090.8A GB9608090D0 (en) | 1996-04-19 | 1996-04-19 | Liquid crystal display cell using phosphor emitters |
GB9608090 | 1996-04-19 | ||
PCT/GB1997/001091 WO1997040416A1 (en) | 1996-04-19 | 1997-04-18 | Liquid crystal display cell using phosphor emitters |
Publications (1)
Publication Number | Publication Date |
---|---|
EP0894283A1 true EP0894283A1 (en) | 1999-02-03 |
Family
ID=10792306
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
EP97919502A Withdrawn EP0894283A1 (en) | 1996-04-19 | 1997-04-18 | Liquid crystal display cell using phosphor emitters |
Country Status (5)
Country | Link |
---|---|
EP (1) | EP0894283A1 (en) |
JP (1) | JP2000508788A (en) |
AU (1) | AU2394797A (en) |
GB (1) | GB9608090D0 (en) |
WO (1) | WO1997040416A1 (en) |
Families Citing this family (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
WO2002019020A1 (en) * | 2000-08-29 | 2002-03-07 | Freelight Systems Limited | Display comprising a fluorescent dye doped polymer |
WO2014065650A1 (en) * | 2012-10-24 | 2014-05-01 | Universiti Sains Malaysia | A method for adding photoluminescent pigment on glass |
Family Cites Families (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
FR2600451A1 (en) * | 1986-06-20 | 1987-12-24 | Morin Francois | Colour display device using a liquid crystal cell and a three-coloured fluorescent screen |
US4830469A (en) * | 1987-02-13 | 1989-05-16 | U.S. Philips Corporation | Liquid crystalline color display cell having a phosphorescent substrate and U.V.-absorbing dichroic dye |
-
1996
- 1996-04-19 GB GBGB9608090.8A patent/GB9608090D0/en active Pending
-
1997
- 1997-04-18 EP EP97919502A patent/EP0894283A1/en not_active Withdrawn
- 1997-04-18 JP JP9537831A patent/JP2000508788A/en active Pending
- 1997-04-18 WO PCT/GB1997/001091 patent/WO1997040416A1/en not_active Application Discontinuation
- 1997-04-18 AU AU23947/97A patent/AU2394797A/en not_active Abandoned
Non-Patent Citations (1)
Title |
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See references of WO9740416A1 * |
Also Published As
Publication number | Publication date |
---|---|
GB9608090D0 (en) | 1996-06-26 |
WO1997040416A1 (en) | 1997-10-30 |
AU2394797A (en) | 1997-11-12 |
JP2000508788A (en) | 2000-07-11 |
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