KR20060136392A - Threshold voltage compensation method for electroluminescent display devices - Google Patents

Abstract

The active matrix electroluminescent display device has a shorting transistor 30 coupled between the gate and the drain of the drive transistor 22. Means 42 are provided for measuring the voltage at data line 6. The shorting transistor 30 may be used to discharge the voltage on the gate of the drive transistor 22 until switching off. By storing the resulting voltage on data line 6 through address transistor 16, the data lines are used as one of the control / measure lines for threshold measurement.

Description

Threshold Voltage Compensation Method for Electroluminescent Display Devices {THRESHOLD VOLTAGE COMPENSATION METHOD FOR ELECTROLUMINESCENT DISPLAY DEVICES}

The present invention relates to an electroluminescent display device, in particular an active matrix display device having a thin film switching transistor associated with each pixel.

Matrix displays using electroluminescent display devices are well known. Display elements may include organic thin film electroluminescent elements, such as polymer materials or other light emitting diodes (LEDs) using traditional III-V inverted composites. Recent developments in organic electroluminescent materials, particularly polymer materials, have demonstrated the ability to be used practically in video display devices. Such materials typically comprise one or more layers of semiconducting conjugated polymer inserted between a pair of electrodes, one of which is transparent and the other is suitable for injecting holes or electrons into the polymer layer. Include.

The polymer material can be made only by a CVD process or spin coating technique using a solution of a composite polymer that can be dissolved. Ink-jet printing can also be used. Organic electroluminescent materials exhibit the same I-V characteristics as diodes, providing both display and switching functions and can be used in passive type displays. Alternatively, these materials can be used in an active matrix display device, each pixel including a display element and a switching device for controlling current through the display element.

Display devices of this type have current-driven display elements, so that conventional analog drive methods include supplying controllable current to the display elements. It is known to provide a gate voltage supplied to a current source transistor that determines current through a display element to a current source transistor as part of the pixel configuration. The storage capacitor maintains the gate voltage after the addressing phase.

1 shows a known pixel circuit for an electroluminescent display device addressed with an active matrix. The display device has a regular array of rows and columns of pixels represented by block 1 and associated switching means located at the intersection between the crossing row (selection) and column (data) address conductors 4,6; Together a panel comprising an electroluminescent display element 2. Only a few pixels are shown in the figure for simplicity. In practice, there may be rows and columns of hundreds of pixels. The pixels 1 are addressed via a row and column address conductor set by a peripheral drive circuit comprising rows, scanning, driver circuit 8 and columns, data, driver circuit 9 connected to the end of each conductor set. Is specified.

The electroluminescent display element 2 is here represented as a diode element (LED) and comprises an organic light emitting diode comprising a pair of electrodes sandwiched between one or more active layers of organic electroluminescent material. The display elements of the array are delivered with associated active matrix circuitry on either side of the insulating support. The cathode or anode of the display element is formed of a transparent conductive material. In order to deliver light generated by the electroluminescent layer through these electrodes and the support for viewing by the viewer on the other side of the support, the support is a transparent material such as glass, the display element closest to the substrate. The electrode of (2) may be composed of a transparent conductive material such as ITO. Typically, the thickness of the organic electroluminescent material layer is between 100 nm and 200 nm. Typical examples of suitable organic electroluminescent materials that can be used for element 2 are known and described in EP-A-0 717446. The composite polymer material described in WO96 / 36959 can be used.

2 shows a known pixel and drive circuit arrangement for providing a voltage-programmed operation in the form of a simplified schematic. Each pixel 1 includes a driving circuit associated with the EL display element 2. The driver circuit has an address transistor 16 on the row conductor 4 which is turned on by a row address pulse. When the address transistor 16 is turned on, the voltage on the thermal conductor 6 can transfer to the remaining pixels. In particular, the address transistor 16 supplies a column conductor voltage to a current source 20 comprising a driver transistor 22 and a storage capacitor 24. A column voltage is provided to the gate of the drive transistor 22 and the gate maintains this voltage by the storage capacitor 24 even after the row address pulse ends. The drive transistor 22 draws current from the power supply line 26.

The drive transistor 22 on this circuit is implemented as a PMOS TFT so that the storage capacitor 24 maintains a fixed gate-source voltage. This results in a fixed source-drain current through the transistor providing the desired current source of the pixel.

One problem, particularly with voltage-programmed pixels using polysilicon thin film transistors, is that different transistor characteristics (especially threshold voltages) on both sides of the substrate cause different relationships between gate voltage and source-drain current and defects in displayed image results Cause.

Various techniques have been proposed to compensate for these threshold voltage variations. Some techniques perform in-pixel measurements of the drive transistor threshold voltage and add this threshold voltage to the pixel drive signal so that the combined drive voltage takes into account the threshold voltage. The pixel circuit to do this requires two storage capacitors, one for the threshold voltage and one for the pixel drive voltage. Further switching transistors are required so that the threshold voltage is measured, for example, by discharging the capacitance across the gate-source of the drive transistor until it is turned off.

Other proposed techniques measure the threshold voltage of the pixel array externally and then adjust the pixel driving signal to compensate for the threshold voltage. These pixel circuits require additional elements so that a signal is supplied to an external circuit to determine the threshold voltage again. For example, it is proposed to measure pixel currents at two driving voltages (saturation region of a driving transistor) and estimate threshold voltages (and mobility) from them. This provides not only more complex pixel circuits but also more complex pixel driving methods.

Although this avoids the need for circuit elements for in-pixel compensation, there remains a need for a simple pixel circuit that provides threshold voltage information to an external measurement circuit with a simple driving method. . Simplification of the pixel circuits reduces manufacturing problems of large displays and improves yield. In addition, the reduction in the number of pixel circuit elements can cause the pixel aperture to increase (depending on the configuration of the pixel circuit), and the reduction in the space required for the pixel circuit increases the resolution.

According to the present invention, there is provided an active matrix electroluminescent display device comprising an array of display pixels, each pixel comprising:

Electroluminescent (EL) display elements;

A driving transistor for driving a current through the display element;

An address transistor for providing a pixel drive signal from the pixel data line to a gate of the drive transistor; And

A shorting transistor coupled between the gate and the drain of the driving transistor;

Wherein the display device further comprises means for measuring a voltage on the data line.

The pixel device is used to discharge the voltage on the gate of the driving transistor until one additional transistor (shorting transistor) is switched off. By storing the resulting voltage on the data line (via the address transistor), the data line is used as one of the control / measure lines for the measurement of the threshold voltage. This reduces the complexity of the pixel.

The EL display element and the driving transistor are preferably connected in series between the first and second power lines, and the voltage on the second power line is preferably switched between two values, one of which turns off the EL display element. It is possible. Again, this limits in particular any additional complexity of the pixel circuit by allowing the common cathode line to be used as one of the control lines for the threshold measurement operation. In the floating mode, the data line can drift to the gate voltage of the drive transistor of the addressed pixel. Thus, the resulting gate voltage is stored in column capacitance that is particularly present on the data line.

Thus, each pixel can be operated in two modes. In the first threshold voltage measurement mode, the display element is disabled, the address transistor is turned on, and the shorting transistor is turned on. The driving transistor current is shorted to the gate and the gate voltage rises to the gate voltage when the transistor is switched off (if it is a p-type device). In the second pixel drive mode, the display element is enabled, the address transistor is turned on and the shorting transistor is turned off. This is the normal drive mode.

During the first, threshold voltage measurement mode, a predetermined voltage is applied to the data line during the first period, the current is driven through the drive transistor, and during the second period the voltage on all data lines is the gate voltage of the drive transistor. It is allowed to drift so as to follow fairly. In this way, the first period ensures that current is supplied through the drive transistor. The second period causes the drive transistor to be turned off as described above. The resulting gate voltage is stored on the data line.

The drive transistor is preferably a polysilicon TFT, such as a p-type low temperature polysilicon TFT.

The storage capacitor is preferably between the gate and the source of the drive transistor.

The invention also provides a method of addressing pixels of an active matrix electroluminescent display device comprising an electroluminescent (EL) display element and a drive transistor for driving a current through the display element, the method comprising:

Disabling the display element;

Applying a first voltage to the data line;

Driving a current through the drive transistor, through a shorting transistor coupled between the gate and the drain of the drive transistor, and through an address transistor coupled between the gate of the drive transistor and the data line;

Causing the data lines to drift electrically;

Measuring a voltage on the data line; And

Changing the data voltage applied to the drive transistor using the voltage measured on the data line.

This method provides for the operation of the device of the present invention.

Disabling the display device includes applying a disable voltage to a terminal of the display device, such as a common cathode terminal.

The method preferably includes enabling the display element and addressing the pixel with the changed data voltage on the data line, wherein the shorting transistor is turned off.

The invention will now be described by way of example with reference to the following figures.

1 shows a known EL display device;

2 is a schematic diagram of a known pixel circuit for current-addressing an EL display pixel using an input driving voltage.

3 is a schematic diagram of a pixel layout for a display device of the present invention.

4 is a timing diagram for the operation of the circuit in FIG.

5 shows one possible design of a column driver circuit for use in the display device of the present invention.

The same reference numerals are used for different numbers for the same component, and descriptions of these components will not be repeated.

The present invention provides a display pixel circuit in which one additional transistor is connected between the gate and the drain of the driving transistor to provide a threshold voltage measurement function outside of the pixel array.

3 shows a pixel device according to the invention. As in the conventional pixel of FIG. 2, the pixel is voltage-addressed and the storage capacitor 24 maintains the voltage on the gate of the drive transistor 22 after the pixel addressing step.

Compared with the standard pixel layout of FIG. 2, the present invention provides one additional shorting transistor 30 connected between the gate and the gate and the drain of the driving transistor 22. This is controlled by an additional control line 32. The present invention also requires that the common cathode terminal 34 be capable of switching between two voltages, as will be apparent from the description of the operation of the circuit below.

The shorting transistor 30 is used to discharge the voltage at the gate of the driving transistor 22 until switching off. This discharging operation involves removing the charge from the storage capacitor 24 until the voltage across the capacitor reaches a threshold voltage. The resulting voltage on the data line is measured through the turned on address transistor.

The operation of the circuit of FIG. 3 will now be described with reference to the timing diagram of FIG. 4. 4 shows only a portion of the address cycle in which the threshold voltage of the drive transistor is measured.

Plot 4 shows the operations of address transistor 16. Prior to (or at the same time as) an address pulse, the cathode line 34 is raised by disabling the display element by ensuring that it is reverse-biased.

The first voltage is applied to the data line 6 during the period 40, which ensures that once the shorting transistor 30 is turned on, current can be driven through the drive transistor 22. As shown in plot 32, when the shorting transistor is turned on, it provides a path from the power supply line 26 to the data line 6 through the drive transistor 22 and through the address transistor 16. As can be seen, the first voltage on the data line 6 can be ground.

Once the flow of current is established through the drive transistor 22, the data line is drifted by placing the data line in a high impedance state. The data line is a column conductor to the column of pixels and is associated with column capacitance.

When the gate voltage of the drive transistor 22 is maintained by the capacitor 24, it still remains conductive, and the path to the drain-source current passes through the shorting transistor 30 and the capacitor 24. This has the effect of reducing the voltage drop across the capacitor 24, which was the difference between the first voltage, eg ground, and the power supply line voltage. When the voltage across the capacitor is discharged to a threshold value (even though the voltage at the gate rises), the driving transistor 22 is switched off, and no further current flows. Thus, a threshold voltage is stored in the capacitor 24, which is transferred to the thermal capacitance.

In practice, the thermal capacitance is charged relatively slowly, and continues to charge until it reaches the power line voltage, just as the drive transistor 22 has a significant sub-threshold current.

The voltage on the data line is measured so that the threshold voltage is determined. In view of the sub-threshold current mentioned above, the data line voltage is measured as soon as the voltage has time to stabilize at the gate voltage corresponding to the switching-off of the drive transistor. This time will be about 1 ms after the data line is allowed to float, and is within the period shown at 42.

Once the threshold voltage is determined, the pixel data voltage to be applied to the pixel is changed. This can be done in the column driver circuit and can be done in the digital or analog domain. It will be readily apparent to those skilled in the art how the pixel data signal can be changed before being applied to the display. In some cases, a field store is required so that all thresholds can be obtained before compensation, otherwise it may be possible to modify the data voltage to be applied immediately after the measurement.

As will now be apparent, the present invention requires only minor modifications to the standard pixel circuit of FIG. In addition to one extra shorting transistor, a switchable common cathode terminal is required.

In addition to the voltage measuring circuit, a high impedance state for the data input line is implemented outside the pixel array, especially in the column drive circuit. Although some or all of the column driver functionality may also be implemented on the same substrate as the pixel array using LTPS processing, this may be on a separate substrate and may be on crystal silicon.

The present invention enables compensation for threshold voltage variations in polysilicon drive transistors (eg, low temperature polysilicon TFTs).

The above circuit uses a p-type driving transistor. Of course there is an equivalent n-type implementation.

Processing of the threshold voltage measurement from the pixel circuit of the present invention can be performed in a variety of ways. The measured threshold voltage can be digitally combined with the pixel data signal before (D / A conversion) or in the analog region. This combination can occur immediately after the threshold voltage specification, so that the delay in providing image data to the display can be kept to a minimum.

5 shows an example of a possible structure for a column driver circuit. This circuit is operable in two modes and is defined by the output switch 40 for each column.

During the sense mode, switch 40 connects column 6 to a sense circuit comprising voltage sense circuit 42. The sense circuit 42 measures the voltage of the column at the end of the sense period. This data is then sent to the frame store 44. The frame store stores the threshold voltages of all the driving TFTs in the display.

During the pixel drive mode, switch 40 connects column 6 to column drive circuit 46. Then, data for the pixel is supplied to the column driver 46 and the frame store 44 supplies the corresponding threshold voltage. They add an adder 48 to provide data including threshold voltages, and the combined signal is passed to the column driver 46. This is an analog implementation, but the measured threshold voltage can be equally digitized to process pixel data in the digital domain.

Threshold measurement can be performed once per frame of image data, so the threshold measurement cycle is part of every addressing step. In this case, the threshold measurement operation precedes the pixel driving operation.

However, critical measurements do not need to perform this frequently because the required compensation is extracted more from changes across the substrate than differential aging. Thus, the threshold measurement is performed at the beginning of the display cycle, eg every time the display is turned on.

The specific voltages applied to the pixel circuits of the present invention have not been described in detail, and the detailed timing requirements have not been detailed as these are all common design parameters for those skilled in the art.

An example of a column driver illustrates a column voltage sense circuit as "means for measuring thermal voltage." This circuit can take many forms and various specific circuits for this purpose will be apparent to those skilled in the art.

Various other modifications will be apparent to those skilled in the art.

The invention is applicable to electroluminescent display devices, in particular active matrix display devices having thin film switching transistors associated with each pixel.

Claims (13)

An active matrix electroluminescent display comprising an array of display pixels, wherein each pixel is Electroluminescent (EL) display elements 2; A drive transistor 22 for driving a current through the display element 2; An address transistor 16 for providing a pixel drive signal from the data line to the gate of the drive transistor 22; And A shorting transistor 30 coupled between the gate and the drain of the driving transistor, Wherein the display device further comprises means for measuring a voltage on the data line. The active matrix electroluminescent display according to claim 1, wherein the EL display element (2) and the driving transistor (22) are connected in series between a first (26) and a second (34) power supply line. The active matrix electroluminescent display according to claim 2, wherein the voltage of the second power line (34) is switchable between two values, one of which causes the EL display element (2) to be turned off. The data input line (6) according to any one of the preceding claims, wherein the data input line (6) is a voltage driving mode for providing a voltage to a pixel connected to the line and a floating mode that can float to the gate voltage of the driving transistor of the addressed pixel. Active matrix electroluminescent display switchable between floating modes. 5. The method of claim 1, wherein each pixel has two modes: A first threshold voltage measurement mode in which a display device is disabled, the address transistor is turned on, and the shorting transistor is turned on; And An active matrix electroluminescent display operable in a second pixel drive mode in which a display element is enabled, the address transistor is turned on, and the shorting transistor is turned off. The voltage of claim 5, wherein a predetermined voltage is applied to the data line such that current is driven through the drive transistor 22 during the first period 40 during the first, threshold voltage measurement mode. During this time, the voltage on the data line (6) is allowed to float to substantially follow the gate voltage of the drive transistor (22). The active matrix electroluminescent display according to any one of claims 1 to 6, wherein the driving transistor (22) is a polysilicon TFT. 8. An active matrix electroluminescent display according to claim 7, wherein the drive transistor (22) is a low temperature polysilicon TFT. 9. An active matrix electroluminescent display according to any one of the preceding claims, further comprising a storage capacitor (24) between the gate and the source of the drive transistor (22). A method of addressing a pixel of an active matrix electroluminescent display device comprising an electroluminescent (EL) display element (2) and a drive transistor (22) for driving a current through the display element (2), the method comprising: , Disabling said display element (2); Applying a first voltage to the data line (6); Through the driving transistor 22, through the shorting transistor 30 connected between the gate and the drain of the driving transistor, and through the address transistor 16 connected between the gate and the data line 6 of the driving transistor. Driving a current; Causing the data line 6 to drift electrically; Measuring a voltage on the data line (6); And Changing the data voltage applied to a drive transistor (22) using the voltage measured on the data line. 12. The method of claim 10, wherein disabling the display element comprises applying a disable voltage to a terminal of the display element. 12. The active matrix electroluminescent display device of claim 11, wherein disabling the display element comprises applying a disable voltage to a terminal 34 of the display element 2 common to all display elements. Pixel addressing method. 13. The method according to any one of claims 10 to 12, comprising the steps of enabling the display element 2 and addressing the pixel with a changed data voltage on the data line with the shorting transistor turned off. Further comprising a pixel addressing method of an active matrix electroluminescent display device.

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