Electric lamp and discharge devices – With luminescent solid or liquid material – Solid-state type
Reexamination Certificate
1998-03-04
2001-04-03
Patel, Nimeshkumar D. (Department: 2879)
Electric lamp and discharge devices
With luminescent solid or liquid material
Solid-state type
C313S506000, C313S507000, C313S509000
Reexamination Certificate
active
06211613
ABSTRACT:
FIELD OF THE INVENTION
The field of the invention relates to the use of organic light emitting diodes in displays.
BACKGROUND TO THE INVENTION
The most popular flat panel display technology currently in use is based on liquid crystal devices, which are effectively light shutters used in combination with illumination sources. One of the particular advantages of liquid crystal displays is that the power required to switch the light shutters on and off is relatively low and if the light source used to provide illumination is an ambient one as used in reflective LCD technology, then overall power consumption for the display can be very low. Therefore the technology has great potential for low power portable applications e.g. personal organisers. There are two problems, however. One is the poor angle of view, though for portable applications this is not typically a severe constraint. More importantly, the ability to get full colour displays at reasonable contrast is very limited in reflective displays.
Organic electroluminescent devices are made from materials that emit light when a suitable voltage is applied across electrodes deposited on either side of the organic material. One class of such materials is semiconductive conjugated polymers which have been described in our earlier U.S. Pat. No. 5,247,190, the contents of which are herein incorporated by reference. The power requirements, however, of conjugated polymer EL devices in common with other emissive technologies are all relative high compared to reflective LCD.
The readability of a display depends on many factors but contrast and brightness are the most important. A display can be bright, but not easily readable because the contrast with respect to the non-display area is poor. This can certainly be the case in high ambient light conditions, where reflection of light from both display and non-display areas reduces the contrast of the display. This problem is most common in dynamic displays, i.e. in those displays where certain areas may be switched on or off depending on the nature of the information to be displayed. Conversely a display with high contrast in high ambient light conditions can be readable at a relatively low brightness.
SUMMARY OF THE INVENTION
The aim of the present invention is to provide improved contrast monochrome and colour displays with low power requirements that are suitable for portable display applications, and therefore solve the problems described above.
The contrast in organic EL displays can be improved, allowing a reduction in brightness requirements and therefore power for a given readability, by the use of a circular polariser.
This is placed in front of a display which has a reflective back electrode surface and light that passes through the polariser and is then reflected off the back electrode surface is then completely absorbed by the polariser. The polariser allows transmission of a particular “handedness” of either left-handed or right-handed, but on reflection from a metallic surface this handedness is reversed - such a polariser placed in front of a metallic reflector will cause the reflector to appear dark (see Hecht and Zajac or other standard text on Optics and Flat-Panel Displays and CRTs ed by L. Tannas, Van Nostrand Reinhold p.45). If all the ambient light can be absorbed in this way, much lower brightnesses are needed by the display for a given readability, thus saving power. The extent to which this can be achieved is limited by principally three effects. Firstly off-axis light will not be completely circularly polarised, and therefore will not be completely absorbed on reflection. However, for many portable applications, reflected ambient light will only be problematic typically to the extent that it is on axis (this is a function of the normal viewing arrangement for hand-held devices). Secondly if there is a depolarising element (e.g. birefringent medium) between the circular polariser and the rear electrode this will reduce the elimination effect. For on axis light thin organic films can be regarded as isotropic and therefore have minimal effect. Finally unwanted reflections from surfaces between the polariser and the rear reflector will not undergo the full elimination effect. Antireflection coatings can be used on the polariser to reduce the reflection at this interface. The other main interface for reflections is between the transparent conductive electrode (normally indium tin oxide—ITO) and the transparent substrate (typically glass or plastic) and the organic layer(s) and the ITO.
The present invention provides an electroluminescent display, comprising: an organic light-emitting device including at least one layer of an organic material for emitting radiation of at least one colour arranged between first and second conductive layers which act as electrodes for said light-emitting device, and having a light reflective surface behind the surface of said at least one light-emissive layer opposed to a transparent viewing surface of said light-emitting device; and a circular polarizer disposed in front of the viewing surface of said light-emitting device.
Preferably, said electroluminescent display further comprises a substrate on which said light-emitting device is disposed.
In one embodiment, said substrate comprises said circular polarizer.
In another embodiment, said circular polarizer is disposed in front of the surface of said substrate opposed to said surface on which said light-emitting device is disposed.
In a yet further embodiment, said circular polarizer is disposed on the surface of said substrate opposed to said surface on which said light-emitting device is disposed.
Preferably, said first conductive layer is light transmissive.
More preferably, said first conductive layer comprises indium—tin oxide.
Preferably, said second conductive layer comprises aluminium.
Preferably, at least one of said first and second conductive layers is patterned.
In one embodiment, said second conductive layer provides said light reflective surface.
In another embodiment, said light-emitting device further includes a layer of a light reflecting material disposed behind the surface of said second conductive layer opposed to said at least one light-emissive layer, said light reflecting layer providing said light reflective surface.
Preferably, said light-emitting device further includes a layer of an insulating material disposed between said second conductive layer and said light reflecting layer.
Preferably, said light reflecting layer comprises aluminium.
Preferably, said insulating layer comprises metal oxide.
More preferably, said insulating layer comprises aluminium oxide.
Preferably, said light-emitting device includes a plurality of layers of organic light-emissive material.
Preferably, said light-emissive material comprises a light-emissive polymer.
Preferably, said substrate comprises a glass or a plastics material.
Preferably, the numerical difference in the refractive indices of the materials of said substrate, said at least one light-emissive layer and said first conductive layer is not more than 0.3.
Preferably, the refractive indices of the materials of said substrate, said at least one light-emissive layer and said first conductive layer are similar.
Preferably, the refractive index of said substrate is from 1.45 to 2.00, the refractive index of said at least one light-emissive layer is from 1.6 to 2.3 and the refractive index of said first conductive layer is from 1.5 to 2.5.
Preferably, the refractive index of said substrate is from 1.45 to 1.65, the refractive index of said at least one light-emissive layer is from 1.6 to 2.3 and the refractive index of said first conductive layer is from 1.5 to 2.5.
Preferably, the refractive index of said substrate is from 1.45 to 1.65, the refractive index of said at least one light-emissive layer is from 1.6 to 2.3 and the refractive index of said first conductive layer is from 1.7 to 2.0.
Preferably, the refractive index of said substrate is about 1.5, the refractive index of said at least one light-emissive layer is ab
Cambridge Display Technology Limited
Haynes Mack
Merchant & Gould P.C.
Patel Nimeshkumar D.
LandOfFree
High contrast electroluminescent displays does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with High contrast electroluminescent displays, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and High contrast electroluminescent displays will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2525199