Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2002-06-21
2003-12-23
Hannaher, Constantine (Department: 2878)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S069000
Reexamination Certificate
active
06667791
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to displays that utilize light switching elements such as organic light emitting diodes (OLEDs) or liquid crystal (LC), and more particularly, to a display in which such light switching elements are integrated onto a backplane or other substrate.
2. Description of the Prior Art
A display includes a plurality of display elements, or picture elements, i.e., pixels, configured in an array. The display elements include a light switching material, that either generates light (or is emissive), e.g., an OLED, or modulates light, e.g., an LC. An OLED pixel may utilize any of a variety of organic materials that emit light when an electric current is applied thereto. An LC display utilizes an inorganic material to modulate light, that is, alter the phase of the light, as a function of an electric field applied across the material.
The following discussion is primarily directed towards the operation of an OLED display. Nevertheless, the concepts described herein relate to displays that utilize either an organic or an inorganic light switching material.
Illumination of an OLED pixel is controlled by a pixel circuit that may include either a source of current or a source of voltage. It is generally recognized that the constant current source provides a greater uniformity of luminance among the pixels of the array. This is because the dependence of luminance upon current tends to be uniform while luminance at a given voltage the various pixels tends to be less uniform.
Passive or conventional matrix driving is being used for low-resolution OLED displays. However, passive drive resolution is presently limited by the OLED technology to about 100-200 rows for 100 candelas/m
2
display brightness levels. Such displays are being developed for applications such as mobile telephones and mobile video equipment. U.S. Pat. No. 6,023,259 to Howard et al. describes a current driver that provides a passive matrix drive current to an OLED.
Control of the luminance of an “on” pixel is commonly achieved by controlling a magnitude of an analog voltage that determines the voltage or current applied to the pixel. A traditional manner of changing a displayed image is for a processor to update a memory for a display controller that periodically and individually addresses each of the pixels of the display, and to turn them “on” (ON) and “off” (OFF) and any luminance level in between as required.
Passive matrix OLED displays are typically small in format, e.g., 100 rows×100 columns. This constraint is due, in part, to the absence of a commercially viable technique for implementing such a display on a backplane or other large substrate material. An active matrix amorphous silicon (a-Si) or a polysilicon (p-Si) backplane typically suffers a thin film transistor (TFT) threshold voltage shift as a function of electrical stress, and it is regarded as suitable only for low current applications as a-Si devices have low mobility, or electron transport, due to drift having units of cm
2
/V-sec, and are better at applying voltages to a capacitor and operating as a voltage switch; e.g. an active matrix LC. Conventional passive matrix displays on glass are format limited to 320 columns by 240 rows and under, even with split column lines with two drivers for each column with dual row scanning. Also, large size passive drive OLED displays have high row and column voltage drops due to high currents required for passive drive operation. For crystalline silicon (x-Si backplanes), the size is limited to about a 1″ diagonal display.
An additional problem when incorporating a plurality of pixel circuits into a display is that of physically distributing the collective elements of the display. That is, the display is a finite area within which the pixels and their accompanying circuitry are confined, yet a constant pitch between pixels must be maintained in order to provide a uniform image.
An OLED display element includes an organic material interposed between a first conductor and a second conductor. A further problem that limits the feasible size of an OLED display relates to the difficulty of providing signal lines, i.e., the first conductor and the second conductor, to form each individual OLED display element. OLED material is damaged by water, and thus it is not suitable for conventional photolithographic patterning with resist techniques that use water.
Prior art large format large size display black plane drive technologies are not suitable for either high resolution or long lifetimes. Crystalline silicon (c-Si) chips that contain suitable drive circuitry are limited in display size to about 0.5 in
2
. Prior art passive or active matrix displays provide connections to the array from the array edges on a display element side of a backplane substrate.
One prior art approach involves a web-based technology that uses many very small c-Si chips each to drive only a few pixels or alpha numeric display segments that are distributed through out the display. This prior art approach is not suited for large high resolution direct view displays since these c-Si chips would be numerous and visible in the display.
Relatively small (<5.3″ diagonal) polysilicon thin film transistors (TFTs) active matrix OLED displays have been recently reported and shown. Several disadvantages exist. First, TFTs have thick gate oxides and relatively low mobility, thus requiring higher gate to source and higher drain to source voltages to be used in order to develop enough current to drive the OLED to desired brightness levels. The higher voltage operation results in higher power consumption. Secondly, TFT threshold and mobility are not stable with usage, and pattern differential aging artifacts will appear. The results of TFT instability stem from the fact that OLED drive current from pixel to pixel will become non-uniform between pixels having different on/off histories. Patterned uniformity differences as low as 1% are troublesome since they can be seen. To date, only video images that tend to somewhat average the usage of each pixel have been publicly shown. Also, TFTs require low duty cycle AC operation to avoid such film degradation mechanisms as charge trapping and bond breaking, which results in threshold voltage shifts and mobility lowering as a function of operating time. AC operation requires additional compensation such as having the TFT gate to source and perhaps even drain to source voltages reversed for an equal amount of time, thus leaving less time for OLED illumination. Since TFT charge trapping time constants are small, charge trapping occurs very quickly and requires voltage reversal at the display's fame rate. The less time allowed for OLED illumination, the higher the driving TFT biases and currents that are needed, and the greater the resulting TFT instabilities. In addition, higher peak currents result in less OLED efficiency, and if high enough, will lead to irreversible OLED film degradation from heating. From a display size and resolution scaling point of view, the higher the pixel content, the smaller the available row scan time, and the worse the rates of degradation. These issues makes TFT backplanes very difficult, if not impossible, for (1) long life, (2) high resolution large displays, and (3) fixed images such as laptop and desk top monitors.
Because of the aforementioned disadvantages, OLED displays have not been as readily commercialized as have many other conventional display technologies.
SUMMARY OF THE INVENTION
The present invention provides for an improved display in which display elements including light switching material are disposed on a backplane or other large substrate. The present invention also provides such a display where the signals are provided to display elements through vias through and on the backplane.
One embodiment of the present invention is a display apparatus. The apparatus includes (1) a substrate, (2) a display element disposed on the substrate, the display element having (a) a first el
Libsch Frank Robert
Sanford James Lawrence
Gabor Otilia
Hannaher Constantine
International Business Machines - Corporation
Ohlandt Greeley Ruggiero & Perle L.L.P.
Trepp Robert M.
LandOfFree
Passive drive matrix display does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Passive drive matrix display, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Passive drive matrix display will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3182071