Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1998-07-22
2001-05-22
Malinowski, Walter J. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S139000
Reexamination Certificate
active
06236440
ABSTRACT:
BACKGROUND
The invention relates to a display device comprising a first substrate having at least one transparent, first picture electrode of a first material, a second substrate having at least one second picture electrode of a second material which, jointly with the picture electrode on the first substrate and an intermediate opto-electronic material, defines a pixel, and means for supplying electric voltages to the picture electrodes.
The invention also relates to a display device comprising a first substrate having at least one transparent, first picture electrode of a first material, a second substrate having a channel plate coated with a dielectric layer, in which a channel provided with channel electrodes, jointly with the picture electrode on the first substrate and an intermediate opto-electronic material, defines a pixel, and means for driving the pixel.
Display devices of this type are generally known and usually comprise a large number of pixels. The first type comprises, for example, LCDs, both of the active and the passive type; the second type comprises plasma-addressed liquid crystal display devices referred to as PALC displays.
Notably when the second picture electrode is reflective, the drive of these display devices appears to be sensitive to the alternating supply of positive and negative voltages across the pixels. This becomes manifest as, for example, flicker. In a picture period, in which a positive voltage is written, the same picture information leads to a different voltage across the pixel than in the subsequent picture period in which a negative voltage is written. At a frame frequency of, for example 60 Hz, this leads to a flicker frequency of 30 Hz, which is clearly noticeable in the picture. The display devices also often suffer from image retention. In this application, reflecting picture electrodes are also understood to mean partially reflecting, semitransparent electrodes as are used in transflective display devices.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the invention to partly or entirely obviate one or more of said problems.
A first embodiment of a display device according to the invention is therefore characterized in that at least one of the picture electrodes is coated with at least one layer of conducting material, such that the difference in work function between the two picture electrodes is decreased.
A second embodiment of a display device according to the invention is characterized in that at least one of the picture electrodes is coated with at least one layer of a material comprising a dipole, such that the difference in work function between the two picture electrodes is decreased.
The embodiment of a display device according to the invention, based on plasma addressing, is characterized in that the picture electrode, the dielectric layer or at least one channel electrode is coated with at least one layer of material modifying the work function.
The invention is based on the recognition that said image-disturbing phenomena (flicker) result from an initial internal DC voltage across the electro-optical layer. In its turn, this voltage gives rise to ion movements which lead to image retention or cause said transient effect.
Said voltage is presumably the result of the difference in work function between the materials of the two picture electrodes, for example ITO for the transparent picture electrode and aluminum for the reflective picture electrode. Such an asymmetrical structure thus leads to an internal DC voltage which, according to the invention, is neutralized by coating one of the picture electrodes with a layer of conducting material or a layer of material comprising a dipole, for example, an organic material having a low electric conductance, such that the difference in work function between the two picture electrodes is decreased. Experiments have proved that the difference in work function, dependent on the materials used, can be reduced to less than 0.25 eV.
Since also PALC display devices have an asymmetrical structure (a liquid crystal layer between a microsheet and an electrode) there is also such an internal DC voltage in this case which, according to the invention, is neutralized by coating the picture electrodes, the microsheet or the channel electrodes with at least one layer of material modifying the work function of the material of said picture electrodes, microsheet, or channel electrodes.
The difference in work function may be decreased in different ways, for example, by coating the transparent picture electrode with a layer comprising at least a thin layer of the material of the reflecting picture electrode, or by coating the reflecting picture electrode with a layer comprising at least a layer of conducting material having substantially the same work function as that of the transparent picture electrode. An example of the latter case is the use of a thin layer of gold on a silicon wafer on which, for example, reflecting aluminum electrodes are realized, while switching elements (transistors) are realized in the subjacent silicon. If the gold is several monolayers thick, there is hardly any lateral conductance and this layer does not need to be split up into separate pixels.
The reflecting picture electrode as well as the transparent picture electrode may be coated with a layer comprising at least a layer of the same conducting material.
Also when using (organic) material with a dipole, which is preferably chosen from the group of polyimides or polyamide acids comprising or not comprising fluorine, the reflecting picture electrode and the transparent picture electrode may be coated with a layer comprising at least a layer of the same (organic) material with a dipole, the material on one of the two picture electrodes having been subjected to, for example, an UV treatment when these electrodes are coated with a layer comprising at least a layer of the same (organic) material with a dipole.
If desired, the layer of material with a dipole may function as a orientation layer.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.
REFERENCES:
patent: 5327272 (1994-07-01), Fujiwara et al.
patent: 5667705 (1997-09-01), Mlyazaki et al.
patent: 5705826 (1998-01-01), Aratani et al.
patent: 5764324 (1998-06-01), Lu et al.
patent: 5838398 (1998-11-01), Ilcisin et al.
patent: 10054995A (1998-02-01), None
Buchel Michael
Cnossen Gerard
Deeben Josephus P. A.
Haberern Kevin W.
Johnson Mark T.
Malinowski Walter J.
U.S. Philips Corporation
Waxler Aaron
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