Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From sulfur-containing reactant
Reexamination Certificate
1999-11-10
2001-11-06
Truong, Duc (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From sulfur-containing reactant
C528S423000, C428S690000, C428S917000, C313S001000, C313S504000, C257S040000
Reexamination Certificate
active
06313261
ABSTRACT:
The present invention relates to novel polymer light emitting diodes, components and uses thereof, to a process for the production thereof and a method for light emission. More particularly the present invention relates to novel polymer light emitting diodes adapted for enhanced efficiency light emission, novel semi-conducting polymer components and uses thereof in displays and the like, to a process for the production thereof and a method for light emission.
Illuminated displays have been in existence for some years now and are advantageous in a wide range of applications. Nevertheless shortcomings are continually being addressed by development of improved illumination techniques. For example cathode ray tubes are currently in operation in applications where their high power consumption, bulk and weight are acceptable. In less demanding applications liquid crystal displays are employed which operate by reflective rather than light emissive means. Such display technologies suffer problems of limited viewing angle, poor contrast and the like.
Semi-conduction light emitting diodes are known, employing conventional inorganic semi-conducting materials for light emission purpose. These provide an excellent viewing angle and contrast, but limited range of colours. Inorganic semi-conductors are generally crystalline materials leading to complicated manufacture and limited area of devices which may be made from them.
Light emitting diodes (LED's) comprise two electrode layers, respectively a hole injecting layer and an electron injecting layer, typically comprising highly doped semi-conductor, metallic or ionic sheet layers, having an inorganic semi-conductor layer therebetween which serves to conduct holes and electrons to a region between both electrodes at which photon emission occurs.
LEDs however are limited in the range of inorganic semi-conducting materials available and the wave length range of light in which they emit, commonly the infra-red range.
Efforts to modify LEDs by use of conjugated semi-conducting polymers have been made, for example, as disclosed in Holmes A. B. et al Synthetic Metals 55-57 (1993) 4031-4040 “Photoluminescence and electroluminescence in conjugated polymeric systems”, however as yet it has not been possible to equal the external quantum efficiencies achieved with inorganic LEDs. A common arrangement employs a singe layer of polyparaphenylenevinylene (PPV) as semi-conducting polymer, which has been found to give an external quantum efficiency defined as number of photons emitted in the form of light detected outside the device, per electron flowing through the device of the order of 0.001% for an aluminium electron injecting contract layer, and of the order of 0.01% for a calcium electron injecting contact layer.
Light emission in organic conjugated polymeric materials may occur by the process of photo luminescence or electro luminescence, as indicated in FIG.
1
and FIG.
2
.
It will be apparent that photo luminescence may take place by photo excitation of a polymer, without need for charge conduction, for example photoluminescence has observed for PPV, with emission of light at a longer wavelength (sometimes referred to as the Stokes shift) than that absorbed. Luminescence efficiency by radiative decay of the singlet excition can be reduced by a variety of competing non-radiative decay processes.
Electroluminescence is elcetrically inducded by light emission. In contract to photoluminescence, it results from respective electron and hole injection causing excitation and negative and positive polaron formation. Coincidence of a negative and positive polaron in a luminescent material causes singlet exciton formation with emission of light. Reduced luminescence efficiency may take place as mentioned for photoluminescence, by migration of polarons to a “trap” or “quench” whereby energy is lost-radiatively, or as a result of the proximity of electrodes.
Doping of semi-conducting polymers to improve their semi-conducting behaviour is not wholly successful due to occurrence of phase separation, need for energising the dopant and the like.
Despite the poor efficiency of polymeric LED's, they have highly significant potential advantages in terms of their processability and ability to be deposited over large areas as high quality, robust and/or flexible thin films, for example enabling construction of flexible, very large area LED's, inherent radiative decay properties, emission range covering the whole range of the spectrum, and chemical tailoring of polymer materials to realise desired properties, to name but a few.
According there is a need for an LED and for improved polymeric semi-conductors having the advantages and versatility available with polymeric semi-conductors, however having improved brightness and external quantum efficiencies greater than currently available, for example of the order of 1000 Cd/m
2
and 0.5% respectively and more, thereby meeting commercial requirements.
We have now surprisingly found that it is possible to provide a semi-conducting polymer LED meeting these requirements in admirable manner. In particular we have found that in provision of such LEDs, the arrangement of a given semi-conducting material comprised in an LED may determine the overall external quantum efficiency of the LED. Moreover we have found that the semi-conducting ability of certain materials may be improved by careful selection of synthetic techniques.
In a first embodiment of the invention there is provided in its broadest aspect a device adapted for light emission comprising a plurality of component layers of which a first outer layer is adapted for electron injection, a second opposing outer layers is adapted for “hole” injection, and one or more intermediate layers arranged therebetween are adapted for charge semi-conduction wherein the intermediate layer(s) comprise at least one semi-conducting polymer adapted for electron transport and/or hole blocking, and at least one semi-conducting polymer adapted for hole transport and/or electron blocking wherein the at least one semi-conducting polymer adapted for electron transport and/or hole blocking comprises polymer selected from a nitrogen and/or sulphur containing polymer which is partially or substantially conjugated. Preferably the at least one semi-conducting polymer adapted for electron transport and/or hole blocking is selected from a conjugated polycyclic in which at least one nitrogen and/or sulphur is a heteroatom comprised within a conjugated heterocyclic system. By this means the invention provides a mechanism which resembles the balanced injection and balanced transport of electrons and holes. Preferably the device is characterised by an attractive external quantum efficiency, as hereinbefore defined, and an attractive brightness, measured as Cd/m
2
. Preferably the device comprises intermediate layer(s) comprised of one or more semi-conducting polymers, the type, purity, concentration and layer thickness whereof are adapted for efficient electron and hole transport in relative manner. Preferably the device comprises semi-conducting polymer of chemical and physical nature adapted for electroluminescence, by polaron formation, migration, coincidence and decay in manner that at least one photon of radiation emission is emitted from the device per 400 electrons injected into the device. It is a particular advantage of the invention that the migration and coincidence of electrons and “holes” may be manipulated, whereby a boundary region for coincidence thereof may be positioned relative to the first and second outer layers in an emissive region in manner to provide enhanced brightness and/or external quantum efficiency. Without being limited to this theory, it would seem that the positioning of the boundary region is a function of the respective degrees of transport of electrons and holes within the intermediate layer(s).
Preferably a device of the invention is characterised by an external quantum efficiency of at least 0.1%, more preferably at least 0.2%, most preferably at least
Dailey Stuart
Monkman Andrew Paul
Rebourt Eymard Gabriel Francois Joseph
Samuel Ifor David William
Jacobson & Holman PLLC
Truong Duc
University of Durham
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