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)

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• 1996
  • 1996-04-19 GB GBGB9608090.8A patent/GB9608090D0/en active Pending
• 1997
  • 1997-04-18 JP JP9537831A patent/JP2000508788A/ja active Pending
  • 1997-04-18 WO PCT/GB1997/001091 patent/WO1997040416A1/en not_active Application Discontinuation
  • 1997-04-18 EP EP97919502A patent/EP0894283A1/de not_active Withdrawn
  • 1997-04-18 AU AU23947/97A patent/AU2394797A/en not_active Abandoned

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