Patent Application
20050093848
The invention relates to a passive addressed matrix display having a plurality of luminescent picture elements arranged in the matrix, a plurality of address buses arranged in rows of the matrix and supplied with a select signal having a low signal level and a high non-select signal level, and a plurality of data buses orthogonal to the address buses, each of said luminescent picture elements comprising a luminescent layer between a first and a second display electrode.
A problem of the above passive addressed matrix arrangement is that unwanted current can flow on the non-selected, reverse-biased, picture elements, whereby the non-selected pixels are charged and discharged without use. This leads to high peak currents during the data bus on switching transients and extra power loss.
This situation is particularly critical in polymer-LED and organic-LED displays, in which the LED layers have a comparatively large capacitance. This is caused by the fact that the LED layers are very thin (of the order of 10-500 nm). Especially in the case of larger displays, the capacitance hampers passive matrix operation, as the displacement currents become too large in comparison with the currents used to generate light in the LEDs.
It is an object of the present invention to overcome the above-mentioned problem and to reduce the capacitive power losses.
This and other objects are achieved by a display of the type mentioned in the opening paragraph, which is characterized in that each luminescent picture element is combined with a de-coupling means connected between a respective one of the first and second display electrodes and a respective one of the address buses and the data buses, and in series with the picture element, for preventing said picture element from charging/discharging when it is in the non-select state and another picture element is in the select state.
By incorporating of the inventive de-coupling means, any unwanted charging/discharging of non-selected picture elements can be prevented.
In a special case, it is avoided to allow a discharge current in the pixel diode reverse direction.
For the de-coupling function, an active switch which is electrically controllable (e.g. by a row signal), like an electromechanical switch, may be used, but preferably a passive de-coupling means is used which does not need a control signal.
The passive de-coupling means may advantageously be in the form of a (Schottky) diode appropriately arranged between the respective bus and a display electrode. The Schottky diode can be implemented as a layer combination comprising a metal layer and a semiconducting polymer layer. The semiconducting polymer layer for the Schottky diode may advantageously have the same composition as the luminescent layers of the picture elements. The diode can be arranged on the anode side or on the cathode side of the LED pixel.
The invention is applicable to common anode and common cathode addressing, voltage-driven and current-driven addressing and can advantageously be applied using micro-display technology, where the display is directly manufactured on a (mono) Si chip.
In all cases, the problem of a bright column with a shorter pixel is avoided.
In conventional passive addressing, it is not necessary to have a de-coupling switching function in series with a pixel. In fact, it is unnatural to do so. Consequently, passive addressed emissive matrix displays with series diodes are unknown up to now.
The combination of a switch in series with a pixel element is well known in active addressed matrix displays; in fact, it is fundamental. In active addressing, a pixel is switched on at a row pulse, and is kept active for at least one frame time, until a subsequent row pulse. In this way, the pixel brightness is always equal to the average display brightness. This means that, per pixel, some (TFT) electronics is added to fulfill the following functions:
U.S. Pat. No. 5,479,280 discloses an active matrix addressed liquid crystal display having two switching means and discharge means per pixel. The second switch is redundant, and has no first priority function, other than to increase processing yield. With this construction, however, capacitive crosstalk is injected, to compensate for the initial crosstalk of the first switch, while leakage is prevented with a series diode. Furthermore, this crosstalk is caused by row switching, while in the case of the invention the main function of the de-coupling diode means (transistor) is isolation during column activation.
U.S. Pat. No. 4,651,149 discloses another active addressed liquid crystal display. In said display arrangement, an extra switch (31 in
Another difference between active and passive addressing is, that for passive addressing the light is generated during many short bright flashes during a frame time; this is done many times per second to avoid flicker effects. Furthermore, the switch function used in the invention is to prevent non-selected pixels from being charged from a basically reverse bias situation up to a basically zero bias situation (one could optionally go up to a low forward bias situation, as long as the material threshold voltage is not reached). This situation is different from the one described in U.S. Pat. No. 5,479,280, where a switch is needed to prevent a selected pixel from being discharged.
The objects and advantages of the invention will be set forth further in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
Some embodiments of the invention will be illustrated with reference to the drawings.
Pc=f×n×Cp×Vp×Vp.
Although the charge can be removed via pixel 1 to generate light, or can be stored temporarily to be used during a subsequent charging up, the non-selected pixels 2 . . . n are charged and discharged without use. This leads to high peak currents during the column on switching transients and extra power loss. Another problem arises when so-called shorts are present in the display, indicated by the dashed short-circuit in pixel n. When this pixel is selected, the column current flows from the column line to the low row n voltage, and no light is generated, resulting in a black pixel. Much more annoying is the situation when one of the other rows is selected: the high row n voltage is forced on to the column line, and a selected pixel is always switched on, irrespective of the data input signal Vc at the external column node. In the Figure, Rc counts for the column resistance.
The problem is that unwanted current can flow on the non-selected reverse-biased pixels, leading to large charge storage and row-row current in case of pixel shorts. The invention solves the above problem by adding a de-coupling means, e.g. a diode or a switch, in series with every pixel, to prevent any unwanted charging/discharging. In a special case, it is avoided to allow a discharge current in the pixel diode reverse direction. Only the switch of the selected row pixel has to be turned on. In the driving scheme above, a simple diode will provide this function, as it automatically turns non-conductive in reverse bias. To reduce the stored charge as much as possible, this extra diode should have a small capacitance compared to the pixel capacitance, and a low on-voltage.
P=f×n×Ce×(Vp+Vd)×(Vp+Vd)
For a good mono Si/Schottky type diode, both Cd and Vd are small compared to Cp and Vp resp. which reduces the power dissipation to
P=f×n×Cd×Vp×Vp
Power dissipation can thus be reduced by a factor Cd/Cp. Compared to a conventional active matrix addressed systems, some further advantages are:
A plan view of the resulting lateral pixel-diode arrangement is shown in
Arranged on a surface of substrate 50 (e.g. of Si) are:
A plan view of the resulting vertical pixel-diode arrangement is shown in
An example of a driving scheme is shown in
The voltages supplied to the four illustrated row electrodes are referred to as 11a-d. As is indicated by the division into time segments, the rows are placed at zero voltage potential one at a time. No modulation of these signals is necessary, as their only purpose is to “release” a particular row electrode at a certain time.
The voltage supplied to one of the four illustrated column electrodes is referred to as 12. As is indicated by the division into time segments, voltage pulses 12a-d of different width are fed to the electrode. The first pulse 12a will coincide with the signal 11a feeding a zero voltage to the upper row electrode, resulting in the LED 13a being activated. The second pulse 12b will similarly cause activation of the LED 12b, and so on. Since the brightness of the LED is primarily determined by the current, it is advisable to use current-driving instead of voltage-driving. Instead of using fixed-current/pulse-width-modulation, the brightness can also be controlled by using fixed-width/current-modulation (“amplitude” modulation). To obtain more grey scales, a combination of pulse-width modulation and pulse-height modulation can be used. The switching voltages for the row and columns may be of the order of 10 V.
It should be noted that many modifications of the above-described preferred embodiments can be realized by the skilled person. For example, other suitable materials can be used for the LED stack or the diodes. Also, the diodes can be arranged in a different way between the electrodes, as long as the intended function is achieved. Furthermore, the invention can be implemented, in principle, on any type of display based on the flow of current between two sets of electrodes, where it is desirable to achieve an improved addressing of the pixels.
In summary, the invention relates to a passive addressed matrix display having a plurality of luminescent picture elements. Driving circuitry having means for selecting rows of pixel elements and means for driving pixels in a row is provided. A de-coupling means is connected to each picture element for preventing said picture element from charging/decharging when it is in a non-select state and a simultaneously driven picture element is in a select state.
| Number | Date | Country | Kind |
|---|---|---|---|
| 02075148.3 | Jan 2002 | EP | regional |
| 02078879.0 | Sep 2002 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB03/00047 | 1/10/2003 | WO |
