Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2002-07-01
2004-12-14
Garrett, Dawn (Department: 1774)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S001200, C428S917000, C313S504000, C313S506000, C313S112000, C257S089000, C257S090000, C257S098000, C427S066000
Reexamination Certificate
active
06830831
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a light emitter for a display for use in electronic products and a method of forming the light emitter and display.
2. Prior Art
Modern consumer electronics require cheap, high-contrast displays with good power efficiency and low drive voltages. Particular applications include displays for mobile phones and hand-held computers.
Conventional displays comprise twisted nematic liquid crystal displays (TN-LCDs) with active matrix addressing and super-twisted nematic liquid crystal displays (STN-LCDs) with multiplex addressing. These however require intense back lighting which presents a heavy drain on power. The low intrinsic brightness of LCDs is believed to be due to high losses of light caused by the absorbing polarizers and filters which can result in external transmission efficiencies of as low as 4%.
SUMMARY OF THE INVENTION
The Applicants have now devised a new kind of light emitter for a display which offers the prospect of lower power consumption and/or higher brightness. The display utilises an alternative light source which can in embodiments be used instead of the conventional polarizers and/or back light. The alternative light source comprises a light emitting polymer or polymer network aligned on a photoalignment layer. The combination of this alternative lighting source with existing LCD technology offers the possibility of low-cost, bright, portable displays with the benefits of simple manufacturing and enhanced power efficiency.
According to one aspect of the present invention there is provided a light emitter for a display comprising a photoalignment layer; and aligned on said photoalignment layer, a light emitting polymer.
The photoalignment layer is comprised of materials that photoalign (e.g. by cross-linking) to form anisotropic layers when polarised light (e.g. UV) is applied.
The photoalignment layer typically comprises a chromophore attached to a sidechain polymer backbone by a flexible spacer entity. Suitable chromophores include cinnamates or coumarins, including derivatives of 6 or 7-hydroxycoumarins. Suitable flexible spacers comprise unsaturated organic chains, including e.g. aliphatic, amine or ether linkages.
An exemplary photoalignment layer comprises the 7-hydroxycoumarin compound having the formula:
Other suitable materials for use in photoalignment layers are described in M. O'Neill and S. M. Kelly, J. Phys. D. Appl. Phys. [2000], 33, R67.
In aspects, the photoalignment layer is photocurable. This allows for flexibility in the angle at which the light emitting polymer (e.g. as a liquid crystal) is alignable and thus flexibility in its polarization characteristics.
The photalignment layer may also be doped with a hole transport compound, that is to say a compound which enables hole transport within the photoalignment layer such as a triarylamine. Examples of suitable triarylamines include those described in C. H. Chen, J. Shi, C. W. Tang,
Macromol Symp
. [1997] 125, 1.
An exemplary hole transport compound is 4,4′,4″-tris[N-(1-napthyl)-N-phenyl-amino]triphenylamine which has the formula:
In aspects, the hole transport compound has a tetrahedral (pyramidal) shape which acts such as to controllably disrupt the alignment characteristics of the layer.
In one aspect, the photoalignment layer includes a copolymer incorporating both linear rod-like hole-transporting and photoactive side chains.
Suitably, the light emitting polymer is a polymer having a light emitting chromophore. Suitable chromophores include fluorene, vinylenephenylene, anthracene and perylene. Useful chromophores are described in A. Kraft, A. C. Grimsdale and A. B. Holmes, Angew. Chem. Int. Ed. Eng. [1998], 37, 402.
Suitably, the light emitting polymer is a liquid crystal which can be aligned to emit polarised light. A suitable class of polymers is based on fluorene.
In one aspect, the light emitting polymer comprises an organic light emitting diode (OLED) such as described in S. M. Kelly, Flat Panel Displays: Advanced Organic Materials, RSC Materials Monograph, ed. J. A. Connor, [2000]; C. H. Chen, J. Shi, C. W. Tang,
Macromol Symp
. [1997] 125, 1; R. H. Friend, R. W. Gymer, A. B. Holmes, J. H. Burroughes, R. N. Marks, C. Taliani, D. D. C. Bradley, D. A. Dos Santos, J. L. Bredas, M. Logdlund, W. R. Salaneck, Nature [1999] 397, 121; M. Grell, D. D. C. Bradley, Adv. Mater. [1999]11, 895; N. C. Greenman, R. H.
Friend Solid State Phys
. [1995] 49, 1.
OLEDs may be configured to provide polarized electroluminescence.
The reactive mesogen (monomer) typically has a molecular weight of from 400 to 2,000. Lower molecular weight monomers are preferred because their viscosity is also lower leading to enhanced spin coating characteristics and shorter annealing times which aids processing. The light emitting polymer typically has a molecular weight of above 4,000, typically 4,000 to 15,000.
The light emitting polymer typically comprises from 5 to 50, preferably from 10 to 30 monomeric units.
The light emitting polymer is aligned on the photoalignment layer. Suitably, the photoaligned polymer comprises uniaxially aligned chromophores. Typically light polarization ratios of 30 to 40 are required, but with the use of a clean up polarizer ratios of 10 or more can be adequate for display uses.
In aspects, the light emitting polymer is formed by a polymerization process. Suitable processes involve the polymerization of reactive mesogens (e.g. in liquid crystal form) via photo-polymerization or thermal polymerization of suitable end-groups of the mesogens. In preferred aspects, the polymerization process results in cross-linking e.g. to form an insoluble, cross-linked network.
The polymerization process can in a preferred aspect be conducted in situ after deposition of the reactive mesogens on the photoalignment layer by any suitable deposition process including a spin-coating process.
In a preferred polymerization process, the light emitting polymer is formed by photopolymerization of reactive mesogens having photoactive end-groups.
Suitable reactive mesogens have the following general structure:
B—S—A—S—B (general formula 1)
wherein
A is a chromophore;
S is a spacer; and
B is an endgroup which is susceptible to radical photopolymerisation.
The polymerisation typically results in a light emitting polymer comprising arrangements of chromophores (e.g. uniaxially aligned) spaced by a crosslinked polymer backbone. The process is shown schematically in
FIG. 1
from which it may be seen that the polymerisation of reactive monomer 10 results in the formation of crosslinked polymer network 20 comprising crosslink 22, polymer backbone 24 and spacer 26 elements.
Suitable chromophore (A) groups have been described previously.
Suitable spacer (S) groups comprise organic chains (e.g. unsaturated), including e.g. flexible aliphatic, amine or ether linkages. Aliphatic spacers are preferred. The presence of spacer groups aids the solubility and lowers the melting point of the light emitting polymer which assists the spin coating thereof.
Suitable endgroups are susceptible to photopolymerization (e.g. by a process using UV radiation, generally unpolarized). Preferably, the polymerization involves cyclopolymerization (i.e. the radical polymerization step results in formation of a cyclic entity).
A typical polymerization process involves exposure of a reactive mesogen of general formula 1 to UV radiation to form an initial radical having the general formula as shown below:
B—S—A—S—B• (general formula 2)
wherein A, S and B are as defined previously and B• is a radicalised endgroup which is capable of reacting with another B endgroup (particularly to form a cyclic entity). The B• radicalised endgroup suitably comprises a bound radical such that the polymerisation process may be sterically controlled.
Suitable endgroups include dienes such as 1,4, 1,5 and 1,6 dienes. The diene functionalities
Contoret Adam Edward Alexander
Kelly Stephen Malcolm
O'Neill Mary
Richards Gary James
Bianco Paul D.
Fleit Martin
Fleit Kain Gibbons Gutman Bongini & Bianco P.L
University of Hull
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