KR20060064614A - Electroluminescent display devices - Google Patents

Abstract

The pixels of an active matrix display device have a current-driven light emitting display element, a drive transistor for driving a current through the display element, a storage capacitor for storing a pixel drive voltage to be used for addressing the drive transistor, a light-dependent device for detecting the brightness of the display element, and driver circuitry for providing data signals to the pixel external to the pixel array. This provides a pixel with optical feedback to compensate for display element ageing. The driver circuitry has a processing means for processing the feedback brightness signals and derives from them a threshold voltage for the drive transistor of the pixel as well as information relating to the performance of the display element, for ageing compensation.

Description

Electroluminescent Display Device {ELECTROLUMINESCENT DISPLAY DEVICES}

The present invention relates to an electroluminescent display device, in particular an active matrix display device having a pixel array comprising a light-emitting electroluminescent display element and a thin film transistor. More specifically, but not exclusively, the present invention relates to an active matrix electroluminescent display device, wherein the pixels of the device are used for responding to light emitted by the display element and for controlling the activation of the display element, It includes a photosensitive element.

Matrix display devices using electroluminescent, light-emitting, display elements are well known. This display device commonly includes an organic thin film electroluminescent device (OLED), which includes a polymer material (PLED), or in addition to a light emitting diode (LED). Such materials typically comprise one or more layers of semiconductive conjugated polymer layer inserted between a pair of electrodes, one of which is transparent and the other suitable for injecting holes or electrons into the polymer layer.

Display elements in such display devices are current driven and conventional analog drive schemes involve providing a controllable current to the display elements. Typically, a current source transistor is provided as part of the pixel configuration, and the gate voltage supplied to the current source transistor determines the current through the electroluminescent (EL) display element. The storage capacitor maintains the gate voltage after the addressing step. Examples of such pixel circuits are described in EP-A-0717446.

Each pixel thus comprises an EL display element and associated driver circuit. The driver circuit has an address transistor, which is turned on by a row address pulse on the row conductor. When the address transistor is turned on, the data voltage on the column conductor can pass through the rest of the pixel. In particular, the address transistor supplies a column conductor voltage to a current source and includes a storage capacitor and a driving transistor connected to the gate of the driving transistor. A column (data) voltage is provided to the gate of the drive transistor, which is held at this voltage by the storage capacitor even after the row address pulse ends. The drive transistor in this circuit is implemented as a p-channel TFT (thin film transistor) so that the storage capacitor keeps the gate-source voltage fixed. This allows the source-drain current through the transistor to be fixed, so that the transistor provides the desired current source operation of the pixel. The brightness of the EL display element is approximately proportional to the current flowing through this element.

In the basic pixel circuit above, differential aging of the LED material or degradation of the LED material, which results in a decrease in the brightness level of the pixel for a given drive current, can cause a change in image quality across the display. Display elements that have been used extensively will be much dimmer than display elements that have been rarely used. In addition, display non-uniformity problems may occur due to the characteristics of the driving transistors, particularly the variability in threshold voltage levels.

Improved voltage-addressed pixel circuits have been proposed that can compensate for aging of LED materials and changes in transistor characteristics. These circuits include a photosensitive element, which is responsive to the light output of the display element and is coupled to the light output to control the integrated light output of the display element for a driving period following the initial addressing of the pixel. In response to act to leak the stored charge on the storage capacitor. Examples of pixel configurations of this type are described in detail in WO 01/20591 and EP 1 096 466. In an exemplary embodiment, when the gate voltage on the driving transistor reaches the threshold voltage, the photodiode in the pixel discharges the gate voltage stored on the storage capacitor and stops the EL display element from emitting, wherein the storage capacitor is Stop discharging. The rate at which charge leaks from the photodiode is a function of the display element output, so that the photodiode acts as a photosensitive feedback device.

In this arrangement, the light output from the display element is independent of the EL display element efficiency, thereby providing aging compensation. Such techniques have been found to be effective in achieving high quality displays, which suffer less non-uniformity over time. However, this method requires a high instantaneous peak brightness level to obtain an appropriate average brightness from a pixel in a frame time, which results in a faster LED material. Since it is easy to age, it is not advantageous for the operation of the display.

In an alternative approach proposed by the applicant, an optical feedback system is used to change the duty cycle in which the display element is operated. The display element is driven at a fixed brightness and optical feedback is used to trigger the transistor switch, which quickly turns off the drive transistor. This avoids the need for high instantaneous brightness levels, but introduces additional complexity to the pixel.

Other improvements that have been proposed for this type of voltage addressed pixel circuitry exist, for example the same for the effect of stress induced threshold voltage variation in the driving transistors that supply current to the EL elements of the pixel. To correct, as described in British Patent Application No. 0305632.2 (PHGB 030025), which makes it possible for amorphous silicon TFTs to be used for driving transistors.

The problem with such pixel circuits is that they add increasing complexity to the pixel circuits and require more components for the pixel circuits, which makes it more difficult to produce high resolution displays.

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

Current-driven light emitting display elements;

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

A storage capacitor for storing a pixel drive voltage to be used to address the drive transistor;

A light-dependent device for detecting the brightness of the display element; And

Driver circuit for providing data signals to pixels outside the pixel array

Wherein the driver circuit further comprises processing means for processing a brightness signal from the light-dependent device of each pixel, wherein the processing means comprises a threshold voltage for the drive transistor of the pixel from a plurality of different brightness signals of each pixel, And derive information relating to the performance of the display element.

In this arrangement, processing of the brightness signal is provided within the driver circuit, which processing is not only to derive the aging compensation scheme but also to obtain the drive transistor threshold voltage. The pixel circuit is thus simplified by transferring at least some of the complexity of the pixel circuit to the driver circuit for the pixel array, so that the pixel circuit contains substantially only the elements necessary for operation. In this way, more complex circuits are accommodated in the drive circuit, preferably in the column drive circuit, not in the pixels themselves.

The brightness signal is preferably present in the form of a large amount of charge stored on a capacitor coupled with the light dependent device. The information relating to the performance of the display element preferably comprises a parameter which takes into account the display element efficiency and the drive transistor mobility.

Each pixel includes a sense transistor for controlling connecting the light-dependent device to a sense line. The light dependent device may be connected in series with a sense transistor between the power supply line and the sense line.

The driver circuit is preferably operable during the setting process to drive the display element of each pixel at a plurality of different predetermined drive levels, wherein the processing means is a light-dependent device of each pixel for each of the plurality of different predetermined drive levels. It is operable to process the brightness signal from.

In addition, analysis of the brightness signal for a number of different and uniform images allows the threshold voltage and aging / mobility parameters to be determined. The setting process may involve driving the display element of each pixel to an off state, a perfect brightness state and an intermediate state. Each pixel may be driven twice to this different level, and difference data is then derived from the data pairs for each pixel for each of a plurality of different predetermined drive levels. This enables compensation for leakage current.

Subsequently, additional difference data may be obtained from the difference data to compensate for the ambient illumination of the light dependent device.

While using the display, a reset operation may be performed on the light dependent device of one pixel or pixel row, and a brightness signal is later obtained from the light-dependent device of one pixel or pixel row. The information is thus obtained in a two stage process. During setup, a complete sensing operation is performed and all parameters are determined. While using a display, some of these parameters can be assumed to be constants, such as the threshold voltage or photodiode efficiency of the polysilicon transistor. During use, simpler sensing operations are performed so that only variable parameters are updated.

During use, the charge may be allowed to increase, and the measurement is made only immediately before the light dependent device of one pixel or one pixel in a row reaches saturation. This may be a plurality of frames after the reset operation, so that the number of read out operations is kept to a minimum and the signal to be finally measured may be large.

The device further includes a memory structure having a first memory region for storing threshold voltage information for the drive transistors of each pixel and a second memory region for storing aging / mobility information for each pixel. can do.

In a preferred embodiment of the present invention, the pixel circuit includes an EL display element, a current source (driving transistor), a memory element, and a switch that allows the pixel to be addressed using a data signal, which is a conventional component. It provides a basic, active matrix pixel circuit and requires minimal components for active matrix operation. The circuit may also further include photosensitive devices such as photodiodes or phototransistors, associated memory elements (capacitors) and additional switches. The photosensitive element uses an associated memory element to sense the brightness of the EL display element, which is converted into an electric charge representing the brightness level and stored in the pixel. This charge can be read from the pixel at any subsequent time, allowing the brightness of the pixel for a given data signal voltage level to be determined. This information can then be used to adjust the input data signal voltage supplied to each pixel to correct the varying TFT threshold voltage and mobility and EL display element efficiency. This correction may be performed in a drive circuit outside the pixel array, preferably in a column drive circuit that supplies a data signal to the pixel.

In another preferred embodiment, each pixel additionally includes another switch for controlling the energization of the display element, involving addressing (selection) and sharing of data signal lines to maximize pixel aperture.

The present invention also provides a method for driving an active matrix display comprising an array of display pixels, each pixel comprising a drive transistor, a current-driven light emitting display element, and a light-dependent device for detecting the brightness of the display element. This method includes:

Driving a display element of each pixel to a plurality of different predetermined drive levels, and processing a brightness signal from the light-dependent device of each pixel for each of the plurality of different predetermined drive levels; And

Deriving from the brightness signal information and threshold voltages relating to the performance of the display device.

It includes. Another aspect of the method of the present invention has been outlined above.

Advantageous features according to the invention are described in detail in the embodiments of various aspects of the invention, which will now be described, by way of example, with reference to the accompanying drawings.

1 is a simplified schematic diagram of an embodiment of an active matrix EL display device.

2 illustrates a known pixel circuit form;

3, 4 and 5 schematically illustrate a pixel circuit in an embodiment of a display device according to the invention.

6 is a diagram used to describe a method of capturing feedback information.

FIG. 7 illustrates a system for implementing the method described with reference to FIG. 6.

8 shows an alternative pixel circuit in an embodiment of a display device according to the invention.

9 schematically illustrates a circuit external to a pixel for adjusting data supplied to a pixel in an embodiment of a display device according to the invention.

Like reference numerals are used to refer to the same or like parts throughout the drawings.

Referring to FIG. 1, an active matrix EL display device comprises a panel having a row and column matrix array of regularly-spaced pixels, referred to as block 10, each pixel being associated with an EL display element 20. And an associated drive circuit for controlling the current through the display element. The pixel is located at the intersection between the row (select) address conductor and the column (data) address conductor, or the intersection set of lines 12 and 14. For simplicity, only a few pixels are shown here. The pixel 10 is addressed via a set of address conductors by a peripheral drive circuit, including a row (scanning) driver circuit 16 and a column (data) driver circuit 18, connected to the ends of each conductor set. All.

Each row of pixels is alternately addressed in a frame period using a selection pulse signal applied to the associated row conductors 12 by the circuitry 16 to program the row pixels with their respective data signals. Each individual display output is determined in a frame period following the address period, and the data signal is supplied by the circuit 18 to the column conductor 14 in parallel. As each row is addressed, the data signal is supplied by the circuit 18 in appropriate synchronization.

The EL display element 20 of each pixel comprises an organic light emitting diode (represented here as a diode element (LED)) and comprises a pair of electrodes, between one or more organic electroluminescent light-emitting materials The active layer is inserted. In this particular embodiment, the material includes a polymer LED material, but other organic electroluminescent materials, such as low molecular weight materials, can be used. The display elements of the array are provided on the surface of the insulating substrate along with the associated active matrix circuitry. The substrate is a transparent material, such as glass, and one of the cathodes or anodes of the display element 20 is formed of a transparent conductive material, such as ITO, so that light generated by the electroluminescent layer is transmitted through the electrode. Typical examples of suitable organic conjugated polymer materials that can be used for EL materials are described in WO 96/36959. Other, low molecular weight, typical examples of organic materials are described in EP-A-0717446.

The drive circuit of each pixel 10 includes a drive transistor, including a low temperature polysilicon TFT (thin film transistor), which drive transistor based on the data signal voltage applied to the pixel via the column conductor 14. It is responsible for controlling the current passing through the display element 20, which is shared by each column of pixels. The column conductor 14 is connected to the gate of the current-control driving TFT through an address TFT in the pixel driving circuit, and the gate for the address TFT of the row pixel is connected to each, common, row address conductor 12.

Although not shown in FIG. 1, each pixel 10 row also shares, in a conventional manner, each power supply line maintained at a predetermined voltage, and a reference potential line, typically provided as a continuous electrode common to all pixels. . The display element 20 and the driving TFT are connected in series between the power supply line and the common reference potential line. For example, the reference potential line may be at ground potential and the power supply line may be at a positive potential, for example near 12V, relative to ground potential.

The features of the display device described so far are generally similar to those of known devices.

2 illustrates a pixel circuit of known type, as described, for example, in WO 01/20591. Here, the driving TFT and the address TFT (both including the p-channel device) are referred to as 22 and 26, respectively, and the power supply line and the reference potential line are referred to as 32 and 30, respectively. When the address TFT 26 is turned on in each row address period by the selection pulse signal applied to the row conductor 12, the voltage (data signal) on the column conductor 14 can be transferred to the rest of the pixel. . In particular, the TFT 26 supplies a thermal conductor voltage to the current source circuit 25, which includes a storage capacitor 24 and a driving TFT 22 connected between the gate of the TFT 22 and the power line 32. . Thus, a column voltage is provided to the gate of the TFT, which is held at this voltage, which constitutes a stored control value, by the storage capacitor 24 even after the address TFT 26 is turned off at the end of the row address period. do. The driving TFT 22 is here implemented as a P-channel TFT and the capacitor holds the gate-source voltage. This causes a fixed source-drain current through the TFT 22, thus providing the desired current source operation of the pixel. The current passing through the display element 20 is regulated by the driving TFT 22 and as a function of the gate voltage on the TFT 22, which gate voltage depends on the storage control value determined by the column voltage, data, and signal. At the end of the row address period, the voltage held by the storage capacitor 24 continues the operation of the display element for the subsequent driving period before the pixel is addressed again in the next frame period. The voltage between the gate of the TFT 22 and the reference potential line 32 thus determines the current passing through the display element 20, and subsequently controls the instantaneous light output level of the pixel.

The known pixel circuit of FIG. 2 further includes a discharge photodiode 34, which is reverse biased, responds to light emitted by the display element 20, and through the photocurrent generated in the photodiode, The light emitted by the device 20 acts to dissipate the charge stored on the storage capacitor 24. The photodiode discharges the gate voltage stored on the capacitor 24, and when the gate voltage on the TFT 22 reaches the threshold voltage of the TFT, the display element 20 will no longer emit light and the storage capacitor will discharge. Stop doing it. The rate at which charge leaks from the photodiode 34 is a function of the display element light output level, so that the photodiode 34 functions as a photosensitive feedback device.

The photodiode feedback arrangement is used to compensate for the degradation effect of display element aging, thereby reducing the operating efficiency of the display element in terms of the light output level generated for a given drive current. Through this degradation, longer and more strongly driven display elements will exhibit reduced brightness, causing display non-uniformity. The photodiode arrangement cancels this effect by maximally appropriately controlling the integrated total light output from the display element in the driving period corresponding to the frame period. The length of time that the display element is activated to generate light during the driving period following the address period is adjusted according to the level of the applied data signal as well as the existing drive current light emission level characteristics of the display element, so that the deterioration effect is reduced. do. A degraded, dimmer, display element will cause the pixel drive circuit to activate the display element for a period longer than that for the brighter, brighter, non-degraded display, so that average brightness is extended to the device operation. It can remain the same over a period of time.

The average light output within the driving period depends on the efficiency of the photodiode 34, which is highly uniform over the pixel array, and is independent of the efficiency of the LED device. However, this output also depends on the threshold voltage of the driving TFT 22, which can vary from pixel to pixel, resulting in display non-uniformity. The pixel circuit of FIG. 2 also requires efficient photodiodes, typically amorphous silicon fin photodiodes, and relatively high peak brightness to achieve reasonable average brightness. Loss of charge stored on storage capacitor 24 also means that the circuit operates at a relatively low brightness level for most driving periods. The circuit thus operates the LED at low efficiency, and thus can lead to increased aging.

3 illustrates an embodiment of the pixel circuit 10 in a display device according to the invention, and more particularly illustrates the basic principle of an external light feedback approach used in the display device. In this pixel circuit, a photosensitive device (in the form of photodiode 40 here) is used again to sense light output from the display element 20 at the pixel. A storage capacitor 44 (separate from the data signal storage capacitor 24) is connected across the photodiode 40 and carries charges generated as a result of light from the display element 20 striking on the photodiode 40. To accumulate. This storage capacitor 44 may be an intrinsic self-capacitance of the photodiode rather than a separate component. The amount of charge stored in the capacitor 24 is determined by the brightness of the pixel and varies accordingly.

The photodiode 40 is connected at one side to a voltage power source, here a power supply line 32, and at the other side to a sense column line 46 via a switching TFT 45, the operation of the switching TFT being It is controlled by a control signal on the read control line 48. This line 48 is shared by all the pixels in the same row so that the switch 45 of all the pixels in the row is actuated simultaneously, while the sense line 46 is shared by all the pixels in the same column. The charge accumulated by the capacitor 44 is read through the line 46 when the switch 45 is activated for circuits outside the pixel array and is measured and used to determine the adjustment to the data signal supplied to the pixel. do. Thus, it will be appreciated that, unlike the circuit of FIG. 2, the light output sensing portion of the pixel circuit is not directly related to the current source portion of the circuit.

The pixel further includes an insulating TFT 36 connected between the display element 20 and the driving TFT 22, which insulates the display element 20 from the driving TFT 22. , Using a switching signal on the control line 48, which may be opened or connected to its gate and shared by other pixels in the same row, to enable the driving TFT to drive the display element and generate a light output. , Can be closed. The switch 45 allows the display element 20 to remain off during the addressing of the pixel using the data signal so that no light is produced during this addressing period and thus no current is induced through the power supply line 32. It doesn't work. This avoids the voltage drop that occurs through line 32 and the resulting crosstalk potential.

In a preferred mode of operation, while the display element 20 is activated by the current supplied by the TFT 22 to produce a light output, the photodiode 40 receives charges resulting from the illumination of the photodiode 40. The switching TFT 45 is kept open to insulate the sensing line 46 so that it can accumulate on the capacitor 44. In this way, beneficial charge signals can be accumulated to make reading simpler. When the switching TFT 45 is closed, the charge reading through the sense line 46 is better than the current, voltage, or signal readout form because of the amplifier circuit connected to the line 46 to provide an indication of the signal level. This is because the sensitivity can be much greater and the operating range can be quite large.

Optical feedback information provided by the circuit of FIG. 3 enables compensation for threshold voltages of drive transistors, mobility of drive transistors, and aging of display elements. As mentioned above, the change in the threshold voltage will cause a shift in the data-voltage / brightness curve and can be adjusted using the shift in the data voltage. Changes in mobility and changes in LED efficiency cause changes in the slope of the curve and require data to be scaled. The change in transistor mobility and the change in LED efficiency work in the same way, and therefore it is their product that matters (so the increase in mobility can accurately counter the reduction in LED efficiency). This product will be referred to below as "apparent mobility."

In order to provide an indication of the mobility and threshold voltage levels of the driving TFTs 22 of the pixels in the array, the display device can be driven to produce a series of (at least two) plain field images at different brightness levels. have. The signal stored on the photodiode storage capacitor 44 of the pixel for each plane field image will then be read by operating switch 45 via sense line 46. This read is made for one pixel row at a time, and the stored charge is read for each row in the order following one of the plane image fields and then again following the next plane image field. As described in detail below, from the data thus obtained from each pixel, the threshold voltage and mobility of the driving TFT 22 of the pixel array can be calculated. Since this value does not change significantly with time in the case of the driving TFT 22 including the polycrystalline silicon type TFT, this operation will only need to be performed very occasionally or possibly only when manufacturing the device. At appropriate intervals over the life of the display device, eg, each time the device is turned on, the device can be arranged to operate with a plane field image that has been corrected for threshold / mobility and the sensed charge is read back. Any irregularities in the display image may then simply be due to the aging effect of the EL material of the display element and may be offset by proper image data correction. In the case of the driving TFT 22 including the amorphous silicon TFT, its characteristics may change as a result of the driving level of the individual pixels over the lifetime of the device, and this readout procedure is then preferably performed more frequently.

4 schematically illustrates a preferred implementation of the pixel circuit of FIG. 3. Here, the same column line 14/46 is used for both supplying the data signal and reading the charge signal from the pixel. In addition, the same row address line is used for both the switching TFTs 45 and 36 of the pixels in the row. This enables an increase in pixel aperture.

FIG. 5 illustrates a modified form of the pixel circuit of FIG. 3, in which the phototransistor 50 is used as a photosensitive device rather than the photodiode 40, the gate of the phototransistor 50 being the display element 20. Is connected to the anode. Switch 45 is of opposite conductivity (p-type) to switch 36 (n-type).

An example will now be described of how to operate the pixel circuit of FIG. 5 described above to capture light feedback data for each pixel in the array. The display device is first activated by addressing the rows of pixels one row at a time, using the appropriate data signal to produce a uniform gray level image from the pixels, and turned off while the switch TFT 36 is addressing. It is then turned on in a subsequent display step to allow for activation and light emission of the display element. Switch 36 is then actuated via switch control line 37 to open in rows. This has the effect of turning off each row of pixels in turn. At the same time as the switching of the switch 37 in each row, the switching TFT 45 is complementary in such a way that the capacitor 44 of one pixel row is connected to the shared column line 14/46 at a time. To work. The stored charge can then be reset in preparation for the sensing procedure. This provides a reset operation for the capacitor 44 and provides a clear starting point for the sensing operation.

When each row stops being addressed, the EL display element of the pixel row is re-illuminated and charge integration starts at the storage capacitor of that pixel. At the end of the predetermined illumination period, the display device is addressed one row at a time to turn off the display element. In this way, the termination of the charge integration can be made precisely without disturbing the stored charge. This charge can then be read slowly in subsequent periods.

Pixels and display devices can be operated in different ways to achieve the same functionality, and alternative pixel circuit schematics can be used.

A number of methods of using the optical feedback information obtained as disclosed above will now be described.

In the first embodiment, feedback information is obtained using pixels operated over six frames (each frame is driven with a uniform image) so that optical feedback information is obtained for different display drive states. As disclosed below, these frames represent three different image brightness levels, with each level having two different integration periods. Six frames have the following conditions:

1.No pixel data (pixels off), sampling after full frame time

2. Maximum pixel data (pixel full on), sampling after full frame time

3. Intermediate pixel data (pixel medium on), sampling after full frame time

4. No pixel data (pixel off), sampling after half frame time

5. Maximum pixel data (pixel pull-on), sampling after half frame time

6. Medium pixel data (pixel medium on), sampled after half frame time

Provides optical feedback information for.

A set of six feedback data values for each of these pixels can be used to derive the compensation scheme for the pixel drive data, which compensates for threshold voltage changes, mobility changes, display device aging, and also leakage current effects. .

By considering the difference in light output for half frame duration and for the entire frame duration, leakage current effects as well as kickback effects can be compensated. Thus, three new values are derived:

7. No pixel data (pixel off), difference between full frame time and half frame time (value of 1-4)

8. Maximum pixel data (pixel pull-on), difference between full frame time and half frame time (value of 2-5)

9. Intermediate pixel data (pixel medium on), the difference between full frame time and half frame time (value of 3-6)

The effect of external light on the photodiode within the pixel can also be considered as a reference value by using the light feedback signal for no pixel data. Thus, two new values are derived, representing the amount of charge stored on the capacitor.

Q1. Maximum pixel feedback data (value of 8-7) using compensation for ambient light

Q2. Intermediate pixel feedback data (value of 9-7) using compensation for ambient light

This calculation not only eliminates the effects of external light, but also the additional leakage current effect. Each of the two values Q1 and Q2 represents the stored charge as data values V1 and V2, using the compensation described above. The two values are the charge values, derived from the measurement of the charge on the photodiode capacitor of each pixel. By enabling readout in the charge region, a high signal-to-noise ratio can be obtained, and the charge can be configured to provide a good signal level.

In a saturated transistor, the drain-source current is:

Is provided.

Given the polymer LED efficiency (η _pl ) and photodiode efficiency (η _pd ), the charge stored over the frame time (T _fr ) is:

Would be the same as

Substituting this equation, two values will be obtained for the charge stored on the capacitor.

K is a constant, taking into account the frame period and the photodiode efficiency, and the two values are substantially constant. This equation contains one term that is the product of transistor mobility and LED efficiency, which is the "obvious mobility" described above. This apparent mobility essentially represents the combined effect of LED aging and transistor mobility variations. These two charge values enable the threshold voltage to be obtained:

The two values also enable the "obvious mobility" to be determined:

Knowing the Q value obtained from the entire display, an average value for the desired display brightness can be found. This can be used to calculate the overall gamma curve of Q for brightness.

The calculation of the parameters as described above enables the correction scheme to be applied to the display. The data voltage to be used for the required Q value (indicative of the desired display brightness) can be calculated using the pixel characteristics as follows:

Thus, this data is moved according to the determined threshold voltage and scaled according to the mobility / aging “obvious mobility” parameter.

The correction scheme requires two optical feedback measurements for different display conditions, and these two degrees of freedom enable the threshold voltage and mobility / aging parameters to be determined. The above approach uses six initial measurements to more accurately derive the two feedback values needed and compensates for various effects. However, a simpler compensation scheme can be used to reach the two feedback values required, depending on the relative leakage current magnitude, ambient light effect and kickback effect. For example, three or four measurements may be sufficient.

In polysilicon drive transistors, the "obvious mobility" will change over time, while the threshold voltage will be relatively stable.

As the threshold voltage can be considered stable in this case, it can be determined during the initial setting procedure and then stored in the frame storage means. Only apparent mobility will move during use. In this regard, a simplified mode of operation can be devised as described below. This mode of operation provides a number of frame measurements as described above during the initial setup procedure or during fabrication, and then more fundamentally while using the display to progressively compensate for changes in mobility and LED aging effects within the driving TFTs. Use charge measurements.

Thus, in manufacturing, the display is operated and measured as outlined above to measure the threshold voltage and apparent mobility (and optionally to remove ambient light effects as well). This value is then stored and the threshold voltage shift value will never change after that.

At the start of use, the display will be uniform without apparent burn-in from LED differential aging. During use, the LED will age and the apparent mobility value will be inaccurate. If the data is moved to compensate for the threshold voltage, a single measurement is sufficient to measure the apparent mobility, which can be used to scale the pixel drive data as appropriate. This can then be done line by line during normal use.

This continuous measurement can be done in a variety of ways, one preferred embodiment is described below.

The pixel row (“n” row) is initially reset to reset its photo-sensor storage capacitor. The display will then operate normally. Thus, data is loaded on a pixel in rows and each row of pixels is illuminated simultaneously. The photo-sensor for the "n" row will sample the brightness and store the charge on the photo-sensor storage capacitor.

When the display is addressed using the brightness data for the next frame, the charge data persists on the light-sensitive storage capacitor. In this way, the photo-sensor integrates over multiple frames, and a larger feedback signal can be obtained than during a single frame measurement.

The drive system can accurately predict the charge stored on the pixels in an "n" row because the system is responsible for the efficiency of photo-sensing and the brightness of each pixel in the row for every frame that has passed since reset. Because I know.

When the drive system determines that the pixels in the " n " row will be saturated after the next frame (using data for the following frame), it sends a read request. This saturation represents a complete charge or discharge of the photo-sensor storage capacitor. In response to the read request, the " n " row continues to receive a read operation at the end of the current frame.

The new feedback data obtained from the "n" row is then used to calculate new apparent mobility data for the pixels on that row, which data is then stored in the previously stored data (at the time of manufacture or at the end of the apparent mobility data). It is used to replace one of the stored data at the time of update.

Obvious mobility data can be derived from a single charge measurement using information from the initial set up measurement and assuming that some parameters such as threshold voltage and photodiode efficiency remain constant.

After the data set for the " n " row is completed, the photodiode capacitor for the " n + 1 " row can be reset, and this operation can continue on a row-by-row basis. Subsequently, for each row, the system calculates and integrates data for the row to monitor when it begins to saturate as above.

This monitoring approach requires reset and read pulses for one pixel row at a time and at intervals of multiple frames. By charging the photodiode capacitance near saturation, the number of read operations is reduced to a minimum, and a larger charge amount is measured, which simplifies reading and data processing.

The read operation can be performed during a field blanking period between successive fields, so that the read operation does not need to affect the drive scheme timing. This is made possible by requiring only a single row pulse during the field blanking period, and the feedback data is collected from different rows during different blanking periods. However, reading in the field blanking period is not essential. The read operation is short and can therefore be done at different times, depending on the drive scheme.

In a variation on this method, a single pixel can be selected randomly or sequentially from the display. This pixel will then be reset, activated, monitored, and read. This results in resetting at least the entire row in which the pixel is located, and reading the entire row, but ignoring information from other pixels in this row.

6 is used to describe the method outlined above. The top figure shows the brightness of a pixel within a given row (n rows) for several frames. This pixel displays video data that changes over time. The middle figure shows the charge that accumulates in the photo-sensor storage capacitor over this time. The rate of increase in charge depends on the pixel brightness, as shown. In this figure, the horizontal dotted line represents the saturated charge of the pixel.

The bottom figure shows the addressing steps of the display. Each addressing step includes a row address pulse for each pixel row, the timing of the row address pulse for n rows is represented by section 60, and the row address pulses for all other rows are stored in section 62. Timing

Pulse 64 represents a read pulse for n rows, and pulse 66 represents a read pulse for n-1 rows.

When one row is read, the next row is reset for the measurement. Thus, pulse 66 not only provides a read for n-1 rows but also resets the photodiode storage capacitor for n rows. Normal data is provided in n rows over the first four frames shown, and the photo-sensor samples the light during each frame. The storage capacitor stores the charge and integrates it over several frames as described above.

The drive system monitors approximately how much charge is stored in the photo-sensor storage capacitor of every pixel in n rows. At the end of the fourth frame, assuming that the pixel shown in FIG. 6 is the pixel with the highest stored charge, the drive system can predict that another frame will saturate the pixel. This saturation is shown in the middle figure of FIG. 6 as a dashed extrapolation line (intersecting the saturation level). This triggers a read pulse 64 over n rows. This read pulse may occur in the field blanking period 68, as shown, after the end of addressing of all rows for this frame. This reads the pixels over n rows, and subsequently the n + 1 rows are reset to ready for integration and measurement over the next few frames.

7 shows one possible structure for the drive system.

Pixel data is input from the brightness data input unit 100. This data is provided to the charge calculation unit 102, which calculates how much charge the photo-sensor will generate over the next frame.

This is subsequently provided to the integrator 104, the value stored by the integrator is monitored by the saturation predictor 106, which estimates whether any pixel will be saturated over the next frame. If saturated, the drive system will read the row at the next opportunity and using the reset and read unit 107 before the data is loaded onto the display.

This data is also provided to the compensation function unit 108, which obtains the correct threshold voltage Vt and mobility data from the frame storage means, and corrects the data value to be compensated.

This corrected data is provided to the line storing means 110 (or frame storing means according to the addressing method) prepared for input to the display. If the row is not read, this data will be delivered to the display in the correct addressing manner.

When a row is read, charge amplifier 112 measures data from every pixel on the row, and passes this data to Vt calculator 114 and mobility calculator 116. These calculators measure the measurement output data from either manufacture or use, the predicted integrated charge for that frame group, and calculate the pixel mobility and threshold voltage. This value is transferred and stored in the respective frame storage means 118, 120 in preparation for subsequent compensation operations.

The functions performed by this system are divided into units in FIG. In fact, the calculations will be implemented in the processor and the drawings are for illustrative purposes only. Data correction can occur entirely in the digital domain, for example using separate drive chips.

The pixel circuit described above is intended to use polycrystalline silicon TFTs. Other variations suitable for using amorphous silicon TFTs are possible. In this case, the sense reading scheme may also be used to compensate for LED material degradation as well as threshold voltage drift in the driving TFT.

8 illustrates a pixel circuit variant suitable for use with amorphous silicon technology. The switch line 37 controls the switch 36, which is here connected in parallel with the driving TFT 22 of the pixel and not in series. The closing of the switch 36 raises the anode of the display element 20 high, ie to the voltage of the power supply line 32, during the addressing step, so that the correct data voltage is across the driving TFT 22. Will be programmed. The phototransistor 50 can be biased by connecting its gate to a suitable node in the pixel circuit, or possibly one pixel in the previous row, depending on the driving scheme used. The pixels then operate in a similar manner to the previous embodiment. In this case, however, the display device is not simply corrected at the manufacturing stage but periodically corrected during use. The correction achieved should now be for both threshold voltage drift and LED material degradation.

FIG. 9 illustrates a device external to a pixel in a display device using one of the pixel circuit embodiments of FIGS. 3, 4, 5 and 8 to perform a necessary correction and to obtain a coordinated data signal for supplying the pixel. An alternative circuit arrangement is outlined. This circuit is preferably and conveniently integrated into the column driver circuit 18.

When the display device is in sense mode and the charge is read via sense line 46 or combined sense line and data line 14/46, the charge for one pixel is transferred using charge sensitive amplifier 70. Is measured. The output of the amplifier is fed to the A / D converter 72 and the resulting digital data indicative of the level of the read charge is stored in the corresponding lookup tables (LUTs) 74 and 76. During display device programming, programming data on the threshold voltage and mobility of the driving TFT 22 and LED material degradation from the respective storage means 80 and 82 connected to the LUTs 74 and 76. The programming data for is combined at 84 and added with pixel data at adder 88, which is obtained from column driver circuit 18 and supplied to input 86 of this correction circuit. The suitably corrected data signal output by the adder 88 is then supplied to the data signal line 14 for supply to the pixel via the D / A converter 90 and the buffer 9.

The reading of the charge from the column line 14/46 and the supply of the data signal to this line are controlled by switches 92 and 94, which are operated alternately.

Each pixel column is associated with a similar correction circuit.

In the case of amorphous silicon TFT pixel circuits, there will only be a set of data, including offsets for both threshold voltage and LED material degradation.

Examples of circuits use polycrystalline and amorphous silicon TFTs, but microcrystalline silicon TFTs may also be used.

The photodiode 40 is preferably a pin device.

By reading this disclosure, other changes will be apparent to those skilled in the art. Such modifications may include other features that are already known in the field of active matrix EL display devices and components for the devices and may be used in place of or in addition to the features already described herein.

The present invention is applicable to electroluminescent display devices, in particular active matrix display devices having pixel arrays comprising photo-emitting electroluminescent display elements and thin film transistors.

Claims (36)

An active matrix display device comprising an array of display pixels, wherein each pixel is A current-driven light emitting display element 20; A drive transistor 22 for driving a current through the display element; A storage capacitor 24 for storing a pixel driving voltage to be used for addressing the driving transistor; A light-dependent device 40 for detecting the brightness of the display element; And Driver circuitry for providing data signals to pixels outside the pixel array Wherein the driver circuit further comprises processing means (114, 116) for processing the brightness signal from the light-dependent device of each pixel, wherein the processing means comprises driving the driving transistor of the pixel from a plurality of different brightness signals of the pixel. An active matrix display device adapted to derive information about a threshold voltage and a performance of the display element. According to claim 1, Each pixel further comprising a sense transistor (45) for controlling the connection of the light-dependent device with the sense line (46). According to claim 1, The light dependent device 40 is connected in series with the sense transistor 45 between the power supply line 32 and the sense line 46. The method according to any one of claims 1 to 3, A storage capacitor (24) is connected between the source and the gate of the drive transistor (22). The method according to any one of claims 1 to 4, The brightness signal is in the form of a large amount of charge stored on a capacitor 44 coupled with the light dependent device. The method according to any one of claims 1 to 5, And wherein the information about the performance of the display element comprises parameters that take into account display element efficiency and drive transistor mobility. The method according to any one of claims 1 to 6, A drive transistor (22) is connected between the power supply line (32) and the display element (20). The method according to any one of claims 1 to 7, And the current driven light emitting display element comprises an electroluminescent display element. The method according to any one of claims 1 to 8, The driving circuit may be operable to drive the display element of each pixel at a plurality of different predetermined drive levels during the setting process, and the processing means may be bright from the light-dependent device of each pixel for each of the plurality of different predetermined drive levels. An active matrix display device operable to process a signal. The method of claim 9, The driving circuit is operable to drive the display element of each pixel to the off state, the full brightness state and the intermediate state during the setting process. The method of claim 10, The driver circuit may be operable to cause the display element of each pixel to drive a plurality of different predetermined drive levels twice during the setting process, and the processing means is for each of the plurality of different predetermined drive levels for two different illumination time periods. An active matrix display device operable to process a brightness signal from a light-dependent device of each pixel. The method of claim 11, wherein The processing means derive the difference data from the data pairs for each pixel for each of a plurality of different predetermined drive levels. The method of claim 12, And processing means derive additional difference data from the difference data to compensate for the ambient illumination of the light dependent device. The method of claim 13, And the processing means derives threshold voltage data and mobility data from the additional difference data for each pixel. The method according to any one of claims 1 to 14, The driver circuit may be operable to perform a reset operation 64, 66 of the light dependent device of the pixel or pixel row while using the display, and subsequently process the brightness signal from the light-dependent device of the pixel or pixel row. An active matrix display device operable to control a processing means for the purpose of control. The method of claim 15, The driver circuit may be operable to control the processing means to process the brightness signal from the light dependent device in the pixel or pixel row immediately before the light dependent device of one pixel in one pixel or row reaches saturation. Matrix display device. The method of claim 16, The driver circuit is operable to control the processing means to process the brightness signal from the light dependent device of the pixel or pixel row after a plurality of frames from the reset operation. The method according to any one of claims 1 to 17, Further comprising a memory structure having first memory means 120 for storing threshold voltage information for the drive transistors of each pixel and second memory means 118 for storing mobility information for each pixel, Active matrix display device. The method of claim 18, The drive transistor (22) is a p-type thin film transistor, wherein data is provided only once to the first memory means during the setting procedure, and data in the second memory means is updated while using the display. The method of claim 18, The drive transistor (22) is an n-type amorphous silicon thin film transistor, wherein data is provided only once to the second memory means during the setting procedure, and data in the first memory means is updated while using the display. An active matrix display comprising an array of display pixels, each display pixel comprising a drive transistor 22, a current-driven light emitting display element 20, and a light-dependent device 40 for detecting the brightness of the display element. As a way to drive, Driving the display element 20 of each pixel to a plurality of different predetermined drive levels, and processing a brightness signal from the light-dependent device of each pixel for each of the plurality of different predetermined drive levels; And Deriving from the brightness signal information and threshold voltages relating to the performance of the display device. And an active matrix display comprising a display pixel array. The method of claim 21, During the setting process, deriving threshold voltage and performance information from the brightness signal; And While using the display, processing the brightness signal from the light dependent device of each pixel to update the performance information and thus compensate for differential pixel aging. The method of claim 22, During the setting process, the display element of each pixel is driven to an off state, a full brightness state and an intermediate state. The method of claim 23, wherein During the setting process, the display element of each pixel is driven twice to a plurality of different predetermined levels, and the brightness signal of the light dependent device of each pixel is processed for each of the plurality of different predetermined driving levels for two different illumination time periods. Wherein the active matrix display comprises a display pixel array. The method of claim 24, Deriving a difference signal from a data pair for each pixel for each of a plurality of different predetermined drive levels. The method of claim 25, Deriving additional difference data from the difference data to compensate for the ambient illumination of the light dependent device. The method of claim 26, For each pixel, further comprising deriving threshold voltage data and mobility data from the additional difference data. The method according to any one of claims 21 to 27, While using the display, performing a reset operation of the light dependent device of the pixel or pixel row and subsequently processing a brightness signal from the light-dependent device of the pixel or pixel row. A method of driving an active matrix display. The method of claim 28, The reset operation is performed during a field blanking period of the display. The method of claim 28 or 29, Immediately before a light dependent device of one pixel in one pixel or row reaches saturation, a brightness signal is processed from the light-dependent device of the pixel or pixel row to drive an active matrix display comprising an array of display pixels. Way. The method of claim 30, The saturation state is predicted by estimating the expected state of the photosensitive device of the pixel according to the display data after the reset operation and the display data of the next frame. 32. The method of claim 30 or 31 wherein 10. A method of driving an active matrix display comprising a display pixel array, wherein a brightness signal is processed from a photo-dependent device of a pixel or pixel row after a plurality of frames from a reset operation. The method according to any one of claims 21 to 32, Storing threshold voltage information for the drive transistors of each pixel in one memory, and storing mobility information for each pixel in a second memory region, wherein the active matrix display comprises an array of display pixels. How to. The method of claim 33, wherein The drive transistor is a p-type thin film transistor and comprises an active matrix display comprising an array of display pixels in which data is provided only once to the first memory area during the setup procedure and data in the second memory area is updated while using the display. How to drive. The method of claim 33, wherein The drive transistor is an n-type amorphous silicon thin film transistor, and an active matrix comprising a display pixel array in which data is provided only once to the second memory region during the setup procedure and data in the first memory region is updated while using the display. How to drive a display. The method according to any one of claims 21 to 34, wherein 10. A method of driving an active matrix display comprising a display pixel array, wherein pixel drive data is changed in view of the most recent threshold voltage and mobility information.

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