Computer graphics processing and selective visual display system – Plural physical display element control system – Segmented display elements
Reexamination Certificate
1997-12-08
2001-10-23
Hjerpe, Richard (Department: 2674)
Computer graphics processing and selective visual display system
Plural physical display element control system
Segmented display elements
C345S082000, C313S504000, C428S690000
Reexamination Certificate
active
06307528
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to organic light-emitting displays, and particularly to improved contrast organic light-emitting displays.
2. Description of the Related Art
Research proceeds on many fronts to develop high-resolution displays, for laptop computers, personal communicators and televisions, for example, that are thin, reliable, and low-cost. One technique being actively pursued makes use of the light-emitting properties of certain organic materials. These materials, such as aluminum tris 8-hydroxyquninoline (Alq
3
) , are sandwiched between anode and cathode electrodes, and emit light when a voltage is applied across the electrodes.
One organic-material-based display is discussed in P. May, “Light-Emitting Polymers: A Technology for Conformable Graphic Displays”, SID 96 Digest, paper 14.4, pp. 192-195. The display is configured as a two-dimensional array of individually addressable pixels which form a passive-matrix display. The display's electrodes are formed as first and second layers of parallel strips, with a continuous sheet of organic material sandwiched between the two layers. The electrode strips of the upper layer are oriented perpendicular to the strips of the lower layer, and pixels are formed at the points where the perpendicular electrodes cross. The perpendicular electrode strips act as addressing lines: light is emitted from an individual pixel when a voltage is applied across its upper and lower electrodes so that a current passes through the organic material sandwiched between them.
This arrangement suffers from several problems, however, including poor contrast, optical and electrical crosstalk, and low brightness. Such a display suffers from poor contrast due to the reflective nature of the upper electrodes: the upper electrode strips serve as cathodes, and are made from magnesium-silver (Mg—Ag) a highly reflective material. Ambient light passing through the transparent substrate bounces off the reflective cathodes, which significantly limits the contrast between the pixel's “on state”, i.e., when the pixel is emitting light, and its “off state”.
Because the organic material is one continuous sheet, light spreads from one pixel region to another, resulting in optical crosstalk. The continuous nature of the organic material also leads to electrical crosstalk between pixels. Further crosstalk problems arise due to the fact that the emitted light travels from the organic material to the viewer through the lower electrode strips and through the substrate, requiring both the lower electrodes and the substrate to be a transparent material. The lower electrodes serve as anodes and are made from indium-tin-oxide (ITO), which has a refractive index of about 2.2. The substrate is typically glass, with a refractive index of about 1.5, and air has an index of 1. Due to these different indices, some light is trapped in both the ITO and glass layers, resulting in optical crosstalk and reduced efficiency.
Displays based on organic light-emitting materials are fabricated on glass substrates, with light emitted through the glass. However, because a glass substrate does not support the fabrication of crystalline silicon transistors (which require a silicon substrate), such displays are limited to the passive-matrix type, which have driving transistors placed at the end of each electrode strip/address line only. Active-matrix displays, on the other hand, which are generally preferred due to their superior brightness, place one or more driving transistors at each pixel; this provides for a greater current flow through and an increased brightness from each addressed pixel.
Some efforts have been directed towards display structures which emit light through a transparent upper electrode, as well as through a glass substrate. These displays, discussed for example in U.S. Pat. No. 4,885,211 to Tang et al. and in G. Gu, “Transparent organic light emitting devices,” Appl. Phys. Lett., vol. 68, No. 19 (1996), pp. 2606-2608, are essentially “see-through” devices. However, as with the glass substrate devices discussed above, the transparent nature of the displays limits the amount of contrast which can be obtained between their pixels' on and off states.
SUMMARY OF THE INVENTION
An organic light-emitting pixel structure is presented which overcomes the problems noted above. The novel pixels are arranged into two-dimensional arrays to form high-contrast passive- and active-matrix displays.
The invention makes use of an organic light-emitting material sandwiched between an anode and a cathode. Improved contrast over the prior art is achieved by reducing the amount of ambient light reflected by the pixel, and by reducing the optical scattering between pixels: two of the inherent drawbacks of prior art designs. Reflection of ambient light is reduced by placing a dark, low-reflectance film between a reflective or transparent substrate and the electrode on the substrate side of the organic material. The dark film acts to absorb and/or trap ambient light incident on the pixel, improving the pixel's contrast between on and off states. Optical scattering is reduced by fabricating a vertical wall around each pixel. The wall extends above the pixel's top electrode, effectively blocking out light emitted from adjacent pixels and preventing the scattering of the pixel's own emitted light.
Light emitted from the invention's organic material preferably leaves the pixel through a transparent upper electrode, rather than through the substrate. This reduces the number of material layers through which emitted light must pass from two to one, which decreases the amount of light trapped due to differing refractive indices and increases the pixel's efficiency. Having the emitted light exit through the upper electrode also allows the substrate to be non-transparent. An active-matrix display can be built on a single silicon substrate, with the crystalline silicon transistors needed to drive the active-matrix fabricated alongside their respective pixels. Other non-transparent substrate materials, such as high thermal conductivity materials like metal or diamond, can also be used.
In the case of an active-matrix display, the walls surrounding the pixels are preferably formed from the drive circuitry itself, with the transistors, capacitors, component interconnections and addressing lines stacked up in the spaces between the pixels to form the walls. Alternatively, metal or dielectric materials can be deposited to create the surrounding walls, particularly for displays which do not have circuit components at each pixel, such as a passive-matrix display. The height of the walls surrounding the pixels is made greater than the pixels themselves to block unwanted light out and to keep locally emitted light in.
The corners of the walls surrounding the pixels also serve to physically partition the organic material, which is deposited as a continuous layer, so that each pixel has its own distinct square of organic material. This optically and electrically isolates the pixels, reducing both electrical and optical crosstalk between pixels.
Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.
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G. Gu, V. Bulovic, P.E. Burrows, S.R. Forrest, M.E. Thompson, “Transparent organic light emitting devices”,Appl. Phys. Lett., 68 (19), May 6, 1996, pp. 2606-2608.
P. May, “14.4: Light-Emitting Polymers: A Technology for Conformable Graphic Displays”,SID 96 Digest, May 1996, pp. 192-195.
R. A. Smith, “Semiconductors”, Chapter 10 Optical and high-frequency effects
Duraiswamy Vijayalakshmi D.
Fatahi-yar M.
Hjerpe Richard
Hughes Electronics Corporation
Sales Michael W.
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