Electric lamp and discharge devices – With luminescent solid or liquid material – Solid-state type
Reexamination Certificate
2000-05-31
2003-06-17
Patel, Ashok (Department: 2879)
Electric lamp and discharge devices
With luminescent solid or liquid material
Solid-state type
C313S113000
Reexamination Certificate
active
06580212
ABSTRACT:
FIELD OF THE INVENTION
One specific class of display devices is those that use an organic material for light emission. Light-emissive organic materials are described in PCT/WO90/13148 and U.S. Pat. No. 4,539,507, the contents of both of which are incorporated herein by reference. The basic structure of these devices is a light-emissive organic layer, for instance a film of a poly(p-phenylenevinylene (“PPV”), sandwiched between two electrodes. One of the electrodes (the cathode) injects negative charge carriers (electrons) and the other electrode (the anode) injects positive charge carriers (holes). The electrons and holes combine in the organic layer generating photons. in PCT/WO90/13148 the organic light-emissive material is a polymer. In U.S. Pat. No. 4,539,507 the organic light-emissive material is of the class known as small molecule materials, such as (8-hydroxyquinolino)aluminium (“Alq3”). In a practical device one of the electrodes is typically transparent, to allow the photons to escape the device.
BACKGROUND OF THE INVENTION
FIG. 1
shows the typical cross-sectional structure of an organic light-emissive device (“OLED”). The OLED is typically fabricated on a glass or plastic substrate
1
coated with a transparent first electrode
2
such as indium-tin-oxide (“ITO”). Such coated substrates are commercially available. This ITO-coated substrate is covered with at least a layer of a thin film of an electroluminescent organic material
3
and a final layer forming a second electrode
4
, which is typically a metal or alloy. Other layers can be added to the device, for example to improve charge transport between the electrodes and the electroluminescent material.
There are several approaches available for the processing of conjugated polymers such as PPV. One approach uses a precursor polymer which is soluble and can therefore be easily coated by standard solution-based processing techniques. Examples of coating techniques include: spin coating, blade-coating, reverse roll coating, meniscus coating, contact/transfer coating, and ink-jet printing. The precursor is then converted in situ by suitable heat treatment to give the fully conjugated and insoluble polymer. Another approach uses directly soluble conjugated polymers which do not require a subsequent conversion stage. Depending on the specific application, one or other of the approaches might be preferred. The precursor polymer approach can be especially useful where subsequent processing might lead to damage of the polymer film if it were directly soluble—such processing may be, for instance, coating with further polymer layers (for example, transport layers or emitting layers of another colour), or patterning of the top electrode. Converted precursor films also have better thermal stability, which is of importance both during fabrication and for the storage and operation of devices at high temperatures.
FIG. 2
illustrates one arrangement for depositing light-emissive polymers by ink-jetting, where a glass sheet
10
is coated with an electrode
11
and light-emissive material
12
can then be deposited by ink-jetting on to the electrode
11
. A second electrode can then be deposited over the light-emissive material. (See, for example PCT/WO98/24271, the contents of which are incorporated herein by reference).
When light is produced in an electroluminescent display or other light emitting device it is emitted in all directions. In a device of the type described above some light is emitted forwards, in a viewing direction, through the transparent electrode to the viewer, whilst some is emitted backwards to the opaque metallic electrode where it is either reflected forwards to the viewer or absorbed. Another portion of the light, the portion that is emitted or scattered to more oblique angles, can be waveguided within the emissive layer or within other layers such as the transparent electrode or charge transport layers. The part of the waveguided light that is not absorbed can eventually reach the edge of the emissive pixel. This light is travelling in a direction roughly normal to the principal viewing direction and will not contribute to the brightness of the device as seen by the viewer (see N. C. Greenham et al., Advanced Materials 6 (1994) p491).
The optical structure of the device, and specifically the thicknesses and refractive indices of the component layers, plays an important role in determining how efficiently it is possible for emitted light to be contained within the plane of the device, and thus move away from the electrically-driven pixel. For example, it is possible for light to be ‘trapped’ in ‘slab waveguide’ modes which propagate within the plane of a device of the type shown in
FIG. 1. A
general condition for waveguiding in a region of material is that the region should have a higher refractive index than the materials on either side of it. The emissive organic semiconducting layers can themselves act as this higher refractive index region, in which waveguiding can occur, since these materials commonly show higher refractive indices than the optically transparent materials, such as inorganic glasses or organic polymers which are used as substrate, cladding or insulating layers. The occurrence of this type of waveguiding has been described in some detail in the context of optically-stimulated gain in structure made with such materials, as described for example in: “Spectral Narrowing in Optically-Pumped Poly(p-phenylenevinylene) films”, G. J. Denton, N. Tessler, M. A. Stevens and R. H. Friend., Adv. Mater. 9, 547-551 (1997), “plastic lasers: comparison of gain narrowing with a soluble semiconducting polymer in waveguides and microcavities”, M. A. Diaz Garcia, F. Hide, B. J. Schwartz, M. D. Mcgehee, M. R. Andersson and A. J. Heeger, Appl. Phys. Lett. 70, 3191-3193 (1997), and “Light amplification in organic thin films using cascade energy transfer”, M. Berggren, A. Dodabalapur, R. E. Slusher and Z. Bao, Nature 389, 466-468 (1997).
One common type of display comprises an array of light-emissive regions that can be controlled as independent pixels to allow a desired pattern to be displayed. The array is normally planar, with the light-emissive regions and their associated electrodes and other circuitry formed on a substrate such as a glass sheet. In a device of this type the obliquely emitted light travels in a direction generally in the plane of the display. In a multi-pixel device the waveguided light can cause further problems by causing cross-talk between pixels and reducing the contrast between emitting and non-emitting pixels.
The present invention aims to at least partially address one or more of these problems.
SUMMARY OF THE INVENTION
According to one aspect of the present invention there is provided a display device comprising: a light-emissive structure including two regions or light-emissive material for emitting light in a viewing direction, the regions being spaced apart in a direction perpendicular to the viewing direction and the light-emissive structure being capable of guiding light emitted from one of the light-emissive regions towards the other emissive region; and a barrier structure located between the light emissive regions for inhibiting the propagation of light guided from the said one of the light-emissive regions to the other light-emissive region.
The barrier structure may be a light-absorbent barrier structure of a light-reflective barrier structure. The barrier structure is preferably capable of redirecting in a viewing direction light emitted from the said first light-emissive region towards the barrier structure. Such light could be emitted directly towards the barrier structure or could be waveguided to the barrier structure.
The barrier structure preferably comprises an electrode for injecting electrical charge into the first light-emissive region.
The barrier structure preferably comprises an electrically insulating formation (which may be light-transmissive) and a light-reflective layer. The light-reflective layer is preferably formed over an upper surface of the
Cambridge Display Technology Ltd.
Finnegan Henderson Farabow Garrett & Dunner LLP
Patel Ashok
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