WO2004088626A1

WO2004088626A1 - Active matrix display devices with modelling circuit located outside the display area for compensating threshold variations of the pixel drive transistor

Info

Publication number: WO2004088626A1
Application number: PCT/IB2004/000923
Authority: WO
Inventors: Jason R. Hector; David A. Fish
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2003-04-01
Filing date: 2004-03-23
Publication date: 2004-10-14
Also published as: TW200421221A; GB0307475D0

Abstract

An active matrix display device having current-driven light emitting display pixels uses a modelling circuit outside the display area for modelling the behaviour of a plurality of the display pixels. The modelling circuit is provided with a pixel drive signal derived from the pixel drive signals for the plurality of display pixels. A transistor characteristic of the drive transistor in the modelling circuit is measured and the pixel drive signals for the plurality of display pixels are modified in response to the measured transistor characteristic. This can be used to provide an average correction for threshold voltage drift, and can thereby enable the use of amorphous silicon drive transistors whilst maintaining a simple pixel circuit.

Description

^AC^TIVE MATRIX DISPLAY DEVICE WITH MODELLING CIRCUIT LOCATED OU_TS_ID_{E T}HE _DISPLA_Y AREA FOR COMPENSATING THRESHOLD VARIATIONS OF THE PIXEL DR_IV_{E TRA}N_SISTO_R

DESCRIPTION

5

This invention relates to active matrix display devices, particularly but not exclusively active matrix electroluminescent display devices having thin film switching transistors associated with each pixel.

0 Matrix display devices employing electroluminescent, light-emitting, display elements are well known. The display elements may comprise organic thin film electroluminescent elements, for example using polymer materials, or else light emitting diodes (LEDs) using traditional lll-V semiconductor compounds. Recent developments in organic electroluminescent materials, 5 particularly polymer materials, have demonstrated their ability to be used practically for video display devices. These materials typically comprise one or more layers of a semiconducting conjugated polymer sandwiched between a pair of electrodes, one of which is transparent and the other of which is of a material suitable for injecting holes or electrons into the polymer layer. 0 The polymer material can be fabricated using a CVD process, or simply by a spin coating technique using a solution of a soluble conjugated polymer. Ink-jet printing may also be used. Organic electroluminescent materials can be arranged to exhibit diode-like l-V properties, so that they are capable of providing both a display function and a switching function, and can therefore 5 be used in passive type displays. Alternatively, these materials may be used for active matrix display devices, with each pixel comprising a display element and a switching device for controlling the current through the display element.

Display devices of this type have current-addressed display elements, so that a conventional, analogue drive scheme involves supplying a 0 controllable current to the display element. It is known to provide a current source transistor as part of the pixel configuration, with the gate voltage supplied to the current source transistor determining the current through the display element. A storage capacitor holds the gate voltage after the addressing phase.

Figure 1 shows a known pixel circuit for an active matrix addressed electroluminescent display device. The display device comprises a panel having a row and column matrix array of regularly-spaced pixels, denoted by the blocks 1 and comprising electroluminescent display elements 2 together with associated switching means, located at the intersections between crossing sets of row (selection) and column (data) address conductors 4 and 6. Only a few pixels are shown in the Figure for simplicity. In practice there may be several hundred rows and columns of pixels. The pixels 1 are addressed via the sets of row and column address conductors by a peripheral drive circuit comprising a row, scanning, driver circuit 8 and a column, data, driver circuit 9 connected to the ends of the respective sets of conductors.

The electroluminescent display element 2 comprises an organic light emitting diode, represented here as a diode element (LED) and comprising a pair of electrodes between which one or more active layers of organic electroluminescent material is sandwiched. The display elements of the array are carried together with the associated active matrix circuitry on one side of an insulating support. Either the cathodes or the anodes of the display elements are formed of transparent conductive material. The support is of transparent material such as glass and the electrodes of the display elements 2 closest to the substrate may consist of a transparent conductive material such as indium tin oxide (ITO) so that light generated by the electroluminescent layer is transmitted through these electrodes and the support so as to be visible to a viewer at the other side of the support. Typically, the thickness of the organic electroluminescent material layer is between 100 nm and 200nm. Typical examples of suitable organic electroluminescent materials which can be used for the elements 2 are known and described in EP-A-0 717446. Conjugated polymer materials as described in WO96/36959 can also be used.

Figure 2 shows in simplified schematic form a known pixel and drive circuitry arrangement for providing voltage-addressed operation. Each pixel 1 comprises the EL display element 2 and associated driver circuitry. The driver circuitry has an address transistor 16 which is turned on by a row address pulse on the row conductor 4. When the address transistor 16 is turned on, a voltage on the column conductor 6 can pass to the remainder of the pixel. In particular, the address transistor 16 supplies the column conductor voltage to a current source 20, which comprises a drive transistor 22 and a storage capacitor 24. The column voltage is provided to the gate of the drive transistor 22, and the gate is held at this voltage by the storage capacitor 24 even after the row address pulse has ended. The drive transistor 22 in this circuit is implemented as an n-type TFT, and the storage capacitor 24 holds the gate-source voltage fixed. This results in a fixed source-drain current through the transistor, which therefore provides the desired current source operation of the pixel. The n-type drive transistor can be implemented using amorphous silicon. The drive transistor can be implemented as a p-type transistor, and this will normally be appropriate for implementation using polysilicon.

In the above basic pixel circuit, for circuits based on polysilicon, there are variations in the threshold voltage of the transistors due to the statistical distribution of the polysilicon grains in the channel of the transistors. Polysilicon transistors are, however, fairly stable under current and voltage stress, so that the threshold voltages remain substantially constant.

There is much interest in implementing amorphous silicon pixel circuits for active matrix LED displays. This is becoming possible as the electrical current requirements for the LED devices is reducing with improved efficiency devices. For example, organic LED devices and solution processed organic LED devices have recently shown extremely high efficiencies through the use of phosphorescence. The variation in threshold voltage is small in amorphous silicon transistors, at least over short ranges over the substrate, but the threshold voltage is very sensitive to voltage stress. Application of the high voltages above threshold needed for the drive transistor causes large changes in threshold voltage, which changes are dependent on the information content of the displayed image. This ageing is a serious problem in LED displays driven with amorphous silicon transistors.

There have been a number of proposals for voltage-addressed pixel circuits which compensate for changes in the threshold voltages of the drive transistors used resulting from ageing. Some of these proposals introduce additional circuit elements into each pixel so that the threshold voltage of the drive transistor can be measured, typically every frame. One way to measure the threshold voltage is to switch on the drive transistor as part of the addressing sequence, and to isolate the drive transistor in such a way that the drive transistor current discharges a capacitor across the gate-source junction of the drive transistor. At a certain point in time, the capacitor is discharged to the point where it holds the threshold voltage of the drive transistor, and the drive transistor stops conducting. The threshold voltage is then stored (i.e. measured) on the capacitor. This threshold voltage can then be added to a data input voltage (again using circuit elements within the pixel) so that the gate voltage provided to the drive transistor takes into account the threshold voltage.

These compensation schemes require more complicated pixel configurations and drive schemes.

According to the invention, there is provided an active matrix display device comprising an array of display pixels within a display area, each pixel comprising a current-driven light emitting display element and a drive transistor for driving a current through the display element, wherein the device further comprises at least one modelling circuit outside the display area for modelling the behaviour of a plurality of the display pixels and comprising a current- driven light emitting display element and a drive transistor, the at least one modelling circuit being provided with a pixel drive signal derived from the pixel drive signals for the plurality of display pixels, wherein the device further comprises: means for measuring a transistor characteristic of the drive transistor of the modelling circuit; and means for modifying the pixel drive signals for the plurality of display pixels in response to the measured transistor characteristic.

In this device, a dummy pixel (or pixels) is used to model the ageing of the pixels of the display, and an appropriate correction is made to the pixel drive signals. This can allow conventional pixels to be employed in the display area, or else can allow a simple modification to the pixel circuit and timing to allow for correction of the drive transistor characteristics. The transistor characteristic may be the transistor threshold voltage. The analysis of the dummy pixel is essentially to enable the gate source voltage necessary for the generation of a given current or currents to be determined. Thus, the modelling can take account of other variations in the transistors, for example variations in mobility.

A single modelling circuit can be for modelling the behaviour of all of the display pixels or else a plurality of modelling circuits are provided, each for modelling the behaviour of a respective sub-set of the display pixels. For example, a sub-set of the display pixels can comprise a row of display pixels.

The pixel drive signal provided to the modelling circuit can be derived from an average of the pixel drive signals supplied to the corresponding plurality of display pixels. Although this does not provide correction for the characteristics of the individual drive transistor characteristics, this correction will be adequate in certain circumstances, for example in TV applications where the average brightness of the pixels over time is fairly uniform.

The average of the pixel drive signals can be obtained by averaging the digital image data for the corresponding plurality of display pixels. This digital data is provided to the column driver circuitry and is readily available for supply to a data processor for correction using the information from the pixel modelling circuit.

Alternatively, the average of the pixel drive signals can be obtained by averaging the drive current supplied to the corresponding plurality of display pixels. In this case, circuitry for measuring the current supplied to the display, or to the different portions of the display, is required. The modelling circuit may for example comprise a scaled version of a pixel circuit of the display. This circuit is already provided for other testing purposes.

As mentioned above, the pixel drive signals can be modified in the column driver circuitry. However, the pixel drive signals for the plurality of display pixels can instead be modified using additional circuitry within each display pixel. For example, a storage capacitor is typically provided between the gate and source of the drive transistor and an address transistor is provided between a column data line and the gate of the drive transistor. Additional circuitry can then be provided in the form of a second address transistor between a second column line and the source of the drive transistor. In this way, the storage capacitor holds a gate source voltage which depends both on the pixel data input and the data on the second column line.

Instead, the additional circuitry may comprise a second storage capacitor, the first and second storage capacitors being in series between the gate and source of the drive transistor. In this arrangement, one capacitor is for the data signal and the other is for the threshold voltage.

The invention also provides a method of driving an active matrix display device comprising an array of display pixels within a display area, each pixel comprising a current-driven light emitting display element and a drive transistor for driving a current through the display element, the method comprising: providing at least one modelling circuit outside the display area for modelling the behaviour of a plurality of the display pixels and comprising a current-driven light emitting display element and a drive transistor, providing the at least one modelling circuit being with a pixel drive signal derived from the pixel drive signals for the plurality of display pixels; measuring a transistor characteristic of the drive transistor of the modelling circuit; and modifying the pixel drive signals for the plurality of display pixels in response to the measured transistor characteristic. The invention will now be described by way of example with reference to the accompanying drawings, in which:

Figure 1 shows a known EL display device;

Figure 2 is a simplified schematic diagram of a known pixel circuit for current-addressing the EL display pixel;

Figure 3 shows a display device of the invention;

Figure 4 shows a circuit used within the device of Figure 3;

Figure 5 shows measurement circuitry associated with the circuit of Figure 4; Figure 6 shows a first pixel circuit for use in the device of Figure 3;

Figure 7 is a timing diagram to explain the operation of the circuit of Figure 6;

Figure 8 shows a second pixel circuit for use in the device of Figure 3; and Figure 9 is a timing diagram to explain the operation of the circuit of

Figure 8.

It should be noted that these figures are diagrammatic and not drawn to scale. Relative dimensions and proportions of parts of these figures have been shown exaggerated or reduced in size, for the sake of clarity and convenience in the drawings.

Figure 3 shows a display device of the invention, and which comprises a display area 30, and row and column driver circuits 8, 9 outside the display area. A test unit 32 is provided, in the form of one or more dummy pixels. These additional pixels, outside the display area 32, are often already provided for testing purposes and are frequently termed Process Control Modules or Test Circuits.

Figure 4 shows one possible example of the dummy pixel design for modelling the behaviour of the pixel circuit of Figure 2. The circuit elements 2, 22, 24 of the dummy pixel may replicate those in the pixels, or else the dummy circuit may comprise a scaled version of the pixel circuit. Thus, the dummy circuit may comprise the parallel connection of several pixels so that they circuit behaves in the same way as the pixel circuit but has larger currents flowing for the same voltages. This is easier to measure than a single pixel circuit. Alternatively, the circuit components can be physically larger, although with all circuit components increased in size by the same factor. The important point is that the circuit behaves in the same way as a pixel circuit. In all cases, the dummy pixel circuit represents the actual pixel circuit with similar components and operation to ensure accurate correction.

The dummy pixel circuit includes an additional sense line 40 and a sense transistor 42 connected between the sense line 40 and the source of the drive transistor 22. The dummy pixel circuit is subjected to drive conditions representing the average drive conditions of the pixel array (or else the average drive conditions for a portion of the array). The dummy circuit is then used to measure the drive transistor threshold voltage. For measuring the drive transistor threshold voltage, the sense line 40 is connected to a virtual earth current sensor 50, shown in Figure 5. This device measures current without allowing any change in the voltage on the sense line 40, so that very small currents can be sensed. The current sensor controls the operation of a ramp voltage generator 52. At the start of each field period of the display, the dummy pixel circuit is used to carry out a threshold voltage measurement operation. During the remainder of the field period, the dummy circuit is driven to a voltage to represent the drive conditions of the pixel of the array.

For the threshold measurement operation, address transistor 16 and the sense transistor 42 are turned on. The gate of the drive transistor 22 is then discharged to the voltage on the data column 6 which at that time is arranged to be less than the threshold voltage of the drive transistor 22, so that it is turned off. The anode of the LED display element 2 is also held at the voltage of the sense line 40, which is ground. The power rail 26 is high. The ramp generator 52 then increases the voltage on the column 6, either linearly or in stepwise manner, for example by increasing the voltage output of a buffer, or by injecting charge to the column. The gate of the drive transistor 22 follows the column voltage until the drive transistor turns on, and current is then injected to the sense line 40 and is detected by the current sensor 42. At this time, the voltage output of the ramp generator is stored and is used as a measure of the threshold voltage of the drive transistor. During the remainder of the field period, a signal is provided to the dummy pixel from the data source 54. During this time, the dummy pixel is driven with an average of the driving conditions of the rest of the display.

The average of the pixel drive signals can be obtained by averaging digitally the digital image data for the corresponding plurality of display pixels. Alternatively, the average of the pixel drive signals can be obtained by averaging the drive current supplied to the corresponding plurality of display pixels. In this case, circuitry for measuring the current supplied to the display, or to the different portions of the display, is required.

The dummy pixel is then driven with this average current value, or else with a scaled version of this current, depending on the circuit components in the dummy pixel. The threshold voltage measurement may be once in each field period, as discussed above, but it may be more frequent.

The measured threshold voltage is then added to the desired data voltage for the respective pixels, either in the analogue or digital domains, for example in the source driver circuit (digitally) or in the pixels themselves (analogue). In this way, the pixel drive signals for the plurality of display pixels are modified in response to the measured threshold voltage of the dummy drive transistor threshold voltage. It should be noted that the driving of the dummy pixel can take into account the compensation carried out for the pixels, so that the ageing of the dummy pixel drive transistor accurately reflects the ageing of the corresponding pixels of the array.

Figure 6 shows a first pixel arrangement which enables the threshold voltage to be added within the pixels.

First and second capacitors Cι and C₂ are connected in series between the gate and source of the drive transistor 22. The data input to the pixel is provided to the drive transistor gate by means of the address transistor 16. This data input charges the first capacitor Ci to the pixel data voltage. The second capacitor C₂ is for storing the drive transistor threshold voltage (as determined by the dummy pixel arrangement).

The junction between the first and second capacitors is connected to an additional line 60 through a third transistor 62. This additional line 60 is for providing the threshold voltage to the pixel.

A fourth transistor 64 is connected between the source of the drive transistor 22 and ground. This is used to act as a drain for current from the drive transistor, without illuminating the display element, particularly during the pixel programming sequence. The storage capacitor may comprise an additional storage capacitor (as in the circuit of Figure 2) or it may comprise the self-capacitance of the display element.

The transistors 16, 62, 64 are controlled by respective conductors which connect lo their gates. As will be seen below, the conductors for transistors 62 and 64 may be shared.

Only the drive transistor 22 is used in constant current mode. All other TFTs 16, 62, 64 in the circuit are used as switches that operate on a short duty cycle. Therefore, the threshold voltage drift in these devices is small and does not affect the circuit performance. The liming diagram is shown in Figure 7. The plots 16, 62, 64 represent the gate voltages applied to the respective transistors. Plot 60 represents the voltage applied to the additional line 60, and the clear part of the plot "DATA" represents the timing of the data signal on the data line 6. The hatched area represents the time when data on the data line 6 is for other rows of pixels. It will become apparent from the description below that data for other rows of pixels can be applied during this time so that data is almost continuously applied to the data line 32, giving a pipelined operation.

The circuit operation is to store the data voltage on Cι, and then store the threshold voltage on C₂ so that the gate-source of the drive transistor 22 is the data voltage plus the threshold voltage.

The circuit operation comprises the following steps. The address transistor 16 is turned on, and the third transistor 62 is turned on. During this time, a ground voltage is provided on the line 60 as shown in plot 60. This connects one side of the capacitor Ci to ground and connects the other side to the data voltage, so that the data voltage is stored on Ci.

The address transistor 16 is then turned off so that the capacitor Ci is floating. The threshold voltage 66 is then provided on line 60 and this charges the second capacitor C₂, the opposite terminal of which is connected to ground through the fourth transistor 64. Finally, the transistors 62 and 64 are turned off, and the drive transistor has the combined voltages of the two capacitors applied across its gate-source junction.

Figure 7 shows that the data only needs to be on the column 6 for a period of time corresponding to the row address pulse for the address transistor 16. The second half of the addressing phase can overlap the first half of the addressing phase for an adjacent row, so that a pipelined address sequence can be used. Thus, the length of the addressing sequence does not imply long pixel programming times, and the effective line time is only limited by the time required to charge the capacitor Ci when the address transistor is on. This time period is the same as for a standard active matrix addressing sequence.

Figure 8 shows a second pixel arrangement which allows the threshold voltage to be added within the pixels. The circuit of Figure 8 is in fact the same as the dummy pixel circuit of Figure 4, but the sense line 40 is replaced with an additional input line 70 and the sense transistor 42 is replaced with an additional input transistor 72. This pixel is driven by charging one side of the storage capacitor 24 to the data voltage, and charging the other side of the storage capacitor 24 to a negative voltage equal in magnitude to the threshold voltage. Thus, the total voltage on the storage capacitor is the data voltage added to the threshold voltage.

Figure 9 shows the timing of operation. The addressing period again has two phases. In the first phase, the input line 72 is at ground, and the capacitor 24 is charged to the data voltage through the address transistor 16. During the second phase, the inverse of the threshold voltage is provided on the line 72.

In the two examples above, the pixel is modified to allow addition of the threshold voltage. This enables the voltages required on the column conductors to be kept within limits, as the addition takes place in the pixel. The threshold voltage may alternatively be added to the pixel drive signal by a capacitive coupling effect, for example in a similar manner to the addition of voltages in the so-called "4 level drive scheme" used with active matrix liquid crystal displays.

As described above, the correction enables compensation of the ageing of the pixel circuit components, in particular the drive transistor. The circuit and method of the invention also provides compensation for temperature variations of the display. The characteristics of amorphous silicon circuits are temperature dependent, and the invention can compensate for this temperature dependency by placing the dummy pixel circuits in an area which is subjected to similar temperature conditions as the pixels of the display. In this way, the temperature in the vicinity of the dummy pixel circuits is representative of the temperature of the active pixel area. Circuits have been shown using only n-type transistors. A number of technologies are possible, for example crystalline silicon, hydrogenated amorphous silicon, polysilicon and even semiconducting polymers. These are all intended to be within the scope of the invention as claimed. The display devices may be polymer LED devices, organic LED devices, phosphor containing materials and other light emitting structures.

There are other ways of implementing in-pixel addition of voltages, and there are also numerous ways of implementing changes to the pixel drive signals before they are provided to the columns, for illuminating conventional pixel designs. The various data processing techniques for implementing the modification of the data in the column driver circuitry has not been described in detail as this will be routine to those skilled in the art. In the examples above, an average illumination value is used as the basis of the correction signal. It will be apparent to those skilled in the art that a more complicated scheme may be employed for determining the required correction. This may, for example, take account not only of the average illumination but also the variance in the illumination values, or indeed other statistical parameters.

It is possible for a single correction signal to be applied to the entire array. However, the correction may be row-by-row, or even on the basis of block areas of the pixel array. This may depend on the nature of the data intended to be displayed by the device.

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

Claims

1. An active matrix display device comprising an array of display pixels within a display area (30), each pixel comprising a current-driven light emitting display element (2) and a drive transistor (22) for driving a current through the display element, wherein the device further comprises at least one modelling circuit (32) outside the display area (30) for modelling the behaviour of a plurality of the display pixels and comprising a current-driven light emitting display element (2) and a drive transistor (22), the at least one modelling circuit being provided with a pixel drive signal derived from the pixel drive signals for the plurality of display pixels, wherein the device further comprises: means (50, 52, 54) for measuring a transistor characteristic of the drive transistor of the modelling circuit; and means for modifying the pixel drive signals for the plurality of display pixels in response to the measured transistor characteristic.

2. A device as claimed in claim 1 , wherein a single modelling circuit is for modelling the behaviour of all of the display pixels.

3. A device as claimed in claim 1, wherein a plurality of modelling circuits are provided, each for modelling the behaviour of a respective sub-set of the display pixels.

4. A device as claimed in claim 3, wherein each sub-set of the display pixels comprises a row of display pixels.

5. A device as claimed in any preceding claim, wherein pixel drive signal provided to the modelling circuit is derived from an average of the pixel drive signals supplied to the corresponding plurality of display pixels.

6. A device as claimed in claim 5, wherein the average of the pixel drive signals is obtained by averaging the digital image data for the corresponding plurality of display pixels.

7. A device as claimed in claim 5, wherein the average of the pixel drive signals is obtained by averaging the drive current supplied to the corresponding plurality of display pixels.

8. A device as claimed in any preceding claim, wherein the modelling circuit comprises a scaled version of a pixel circuit of the display.

9. A device as claimed in any preceding claim, further comprising column driver circuitry (9) providing analogue output voltages for driving the pixels of the array, and wherein the means for modifying the pixel drive signals for the plurality of display pixels modifies the analogue output of the column driver circuitry (9).

10. A device as claimed in any one of claims 1 to 8, wherein the means for modifying the pixel drive signals for the plurality of display pixels comprises additional circuitry within each display pixel.

11. A device as claimed in claim 10, wherein each pixel comprises a storage capacitor (24) between the gate and source of the drive transistor (22) and an address transistor (16) between a column data line (6) and the gate of the drive transistor (22), and wherein the additional circuitry comprises a second address transistor (72) between a second column line (70) and the source of the drive transistor (22).

12. A device as claimed in claim 10, wherein each pixel comprises a first storage capacitor (C1) and an address transistor (16) between a column data line (6) and the gate of the drive transistor (22), and wherein the additional circuitry comprises a second storage capacitor (C2), the first and second storage capacitors (C1 , C2) being in series between the gate and source of the drive transistor (22).

13. A method of driving an active matrix display device comprising an array of display pixels within a display area, each pixel comprising a current-driven light emitting display element (2) and a drive transistor (22) for driving a current through the display element, the method comprising: providing at least one modelling circuit (32) outside the display area for modelling the behaviour of a plurality of the display pixels and comprising a current-driven light emitting display element (2) and a drive transistor (22); providing the at least one modelling circuit being with a pixel drive signal derived from the pixel drive signals for the plurality of display pixels; measuring a transistor characteristic of the drive transistor of the modelling circuit; and modifying the pixel drive signals for the plurality of display pixels in response lo the measured transistor characlerislic.

14. A method as claimed in claim 13, wherein the transistor characteristic comprises the threshold voltage.

15. A method as claimed in claim 13 or 14, wherein a single modeling circuit is used for modeling the behaviour of all of the display pixels.

16. A method as claimed in claim 13 or 14, wherein a plurality of modeling circuits are provided, each for modeling the behaviour of a respective sub-set of the display pixels.

17 A method as claimed in any one of claims 13 to 16, wherein the pixel drive signal provided to the modeling circuit is derived from an average of the pixel drive signals supplied to the corresponding plurality of display pixels.

18. A method as claimed in claim 17, wherein the average of the pixel drive signals is obtained by averaging the digital image data for the corresponding plurality of display pixels.

19. A method as claimed in claim 17, wherein the average of the pixel drive signals is obtained by averaging the drive current supplied to the corresponding plurality of display pixels.

20. A method as claimed in any one of claims 13 to 19, wherein the modifying the pixel drive signals for the plurality of display pixels comprises adding a voltage to the pixel drive signals using additional circuitry within each display pixel.