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WO2006013539A1 - Active matrix display devices - Google Patents

Active matrix display devices Download PDF

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Publication number
WO2006013539A1
WO2006013539A1 PCT/IB2005/052511 IB2005052511W WO2006013539A1 WO 2006013539 A1 WO2006013539 A1 WO 2006013539A1 IB 2005052511 W IB2005052511 W IB 2005052511W WO 2006013539 A1 WO2006013539 A1 WO 2006013539A1
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WO
WIPO (PCT)
Prior art keywords
voltage
transistor
pixel
drive transistor
input
Prior art date
Application number
PCT/IB2005/052511
Other languages
French (fr)
Inventor
Mark J. Childs
David A. Fish
Original Assignee
Koninklijke Philips Electronics N.V.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Koninklijke Philips Electronics N.V. filed Critical Koninklijke Philips Electronics N.V.
Publication of WO2006013539A1 publication Critical patent/WO2006013539A1/en

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Classifications

    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
    • G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
    • G09G3/22—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources
    • G09G3/30—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels
    • G09G3/32—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED]
    • G09G3/3208—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED] organic, e.g. using organic light-emitting diodes [OLED]
    • G09G3/3225—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED] organic, e.g. using organic light-emitting diodes [OLED] using an active matrix
    • G09G3/3233—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED] organic, e.g. using organic light-emitting diodes [OLED] using an active matrix with pixel circuitry controlling the current through the light-emitting element
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2300/00—Aspects of the constitution of display devices
    • G09G2300/04—Structural and physical details of display devices
    • G09G2300/0439—Pixel structures
    • G09G2300/0465—Improved aperture ratio, e.g. by size reduction of the pixel circuit, e.g. for improving the pixel density or the maximum displayable luminance or brightness
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2300/00—Aspects of the constitution of display devices
    • G09G2300/08—Active matrix structure, i.e. with use of active elements, inclusive of non-linear two terminal elements, in the pixels together with light emitting or modulating elements
    • G09G2300/0809—Several active elements per pixel in active matrix panels
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2300/00—Aspects of the constitution of display devices
    • G09G2300/08—Active matrix structure, i.e. with use of active elements, inclusive of non-linear two terminal elements, in the pixels together with light emitting or modulating elements
    • G09G2300/0809—Several active elements per pixel in active matrix panels
    • G09G2300/0819—Several active elements per pixel in active matrix panels used for counteracting undesired variations, e.g. feedback or autozeroing
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2300/00—Aspects of the constitution of display devices
    • G09G2300/08—Active matrix structure, i.e. with use of active elements, inclusive of non-linear two terminal elements, in the pixels together with light emitting or modulating elements
    • G09G2300/0809—Several active elements per pixel in active matrix panels
    • G09G2300/0842—Several active elements per pixel in active matrix panels forming a memory circuit, e.g. a dynamic memory with one capacitor
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2300/00—Aspects of the constitution of display devices
    • G09G2300/08—Active matrix structure, i.e. with use of active elements, inclusive of non-linear two terminal elements, in the pixels together with light emitting or modulating elements
    • G09G2300/0809—Several active elements per pixel in active matrix panels
    • G09G2300/0842—Several active elements per pixel in active matrix panels forming a memory circuit, e.g. a dynamic memory with one capacitor
    • G09G2300/0861—Several active elements per pixel in active matrix panels forming a memory circuit, e.g. a dynamic memory with one capacitor with additional control of the display period without amending the charge stored in a pixel memory, e.g. by means of additional select electrodes
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2320/00—Control of display operating conditions
    • G09G2320/02—Improving the quality of display appearance
    • G09G2320/0209—Crosstalk reduction, i.e. to reduce direct or indirect influences of signals directed to a certain pixel of the displayed image on other pixels of said image, inclusive of influences affecting pixels in different frames or fields or sub-images which constitute a same image, e.g. left and right images of a stereoscopic display
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2320/00—Control of display operating conditions
    • G09G2320/02—Improving the quality of display appearance
    • G09G2320/0233—Improving the luminance or brightness uniformity across the screen
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2320/00—Control of display operating conditions
    • G09G2320/04—Maintaining the quality of display appearance
    • G09G2320/043—Preventing or counteracting the effects of ageing

Definitions

  • 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.
  • 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 IN-V semiconductor compounds.
  • LEDs light emitting diodes
  • Recent developments in organic electroluminescent materials, 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.
  • 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 exhibit diode-like I-V properties, so that they are capable of providing both a display function and a switching function, and can therefore 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-driven display elements, so that a conventional, analogue drive scheme involves supplying a 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 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.
  • 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.
  • FIG. 2 shows in simplified schematic form a known pixel and drive circuitry arrangement for providing voltage-programmed 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.
  • 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 draws a current from the power supply line 26.
  • LTPS low temperature polysilicon
  • the threshold voltage of these devices is stable in time, but varies from pixel to pixel in a random manner. This leads to unacceptable static noise in the image.
  • Many circuits have been proposed to overcome this problem.
  • each time the pixel is addressed the pixel circuit measures the threshold voltage of the current-providing TFT to overcome the pixel-to-pixel variations.
  • the use of a-Si:H is now also being considered.
  • 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.
  • each pixel has first and second capacitors connected in series between the gate and source or drain of the drive transistor.
  • the drive transistor threshold voltage is stored on the first capacitor, and the second capacitor is charged to the pixel data voltage.
  • This pixel arrangement enables a threshold voltage to be stored each time the pixel is addressed.
  • a disadvantage of this pixel arrangement is the need for two relatively large capacitors, and the circuit also requires three address lines, to control the various phases of operation of the pixel circuit.
  • an active matrix device comprising an array of display pixels, each pixel comprising: a current driven light emitting display element; a drive transistor for driving a current through the display element from a power supply line; a storage capacitor connected between a data input to the pixel and the gate of the drive transistor; a discharge transistor connected between the gate and drain of the drive transistor; a coupling transistor connected between the data input to the pixel and the power supply line, wherein the pixel is operable to store a voltage on the storage capacitor which is dependent both on an input voltage supplied to the data input and on the drive transistor threshold voltage.
  • This arrangement uses a single capacitor to store a voltage derived both from the data input and the threshold voltage.
  • the threshold voltage is sampled by discharging the capacitor using a discharge transistor connected between the gate and drain. This sampling operation results in the drive transistor turning off.
  • capacitive coupling is used through the capacitor, and a coupling transistor is used to change the voltage at the data input to the pixel. This change is then coupled through the capacitor to the gate so give the desired drive transistor gate voltage.
  • Each pixel preferably further comprises an input first transistor connected between a data line and the data input to the pixel.
  • the input first transistor and the coupling transistor can be controlled in complementary manner, by a shared control line.
  • each pixel further comprises a third transistor connected between the drain of the drive transistor and the display element. This enables the voltage on the drain of the drive transistor to be isolated from the display element when the threshold sampling is taking place.
  • This third transistor can be controlled by the shared control line, so that the circuit can be implemented with only two control lines.
  • the drive transistor preferably comprises an LTPS p-type transistor, although the invention can also be used for amorphous silicon circuits.
  • the display element preferably comprises an electroluminescent (EL) display element.
  • EL electroluminescent
  • the invention also provides a method of driving an active matrix display device comprising an array of current driven light emitting display pixels, each pixel comprising an display element and a drive transistor for driving a current through the display element from a power supply line, the method comprising, for each pixel: applying an input voltage level derived from a desired pixel data voltage to one terminal of a storage capacitor, the other terminal of the storage capacitor being connected to the gate of the drive transistor; turning on the drive transistor; discharging the storage capacitor using the drive transistor until the drive transistor turns off, the one terminal of the first capacitor then carrying the input voltage level and the other terminal of the storage capacitor then carrying a voltage dependent on the threshold voltage; and isolating the input voltage from the one terminal of the storage capacitor and coupling the one terminal of the storage capacitor to the power supply line.
  • This method enables a single capacitor to be used to store a voltage combining a desired pixel drive voltage and a threshold voltage.
  • capacitive coupling of the change in voltage on the one terminal preferably causes the other terminal of the capacitor to carry a voltage which differs from the power supply line by the summation of the threshold voltage and the desired pixel data voltage.
  • the input voltage level is preferably greater than the power supply voltage, for example greater than the power supply voltage by the desired pixel data voltage
  • Figure 1 shows a known EL display device
  • Figure 2 is a schematic diagram of a known pixel circuit for current- addressing the EL display pixel using an input drive voltage
  • Figure 3 shows a schematic diagram of a first example of pixel layout for a display device of the invention
  • Figure 4 is a timing diagram for a first method of operation of the pixel layout of Figure 3;
  • Figure 5 shows a schematic diagram of a second example of pixel layout for a display device of the invention.
  • Figure 6 is a timing diagram for a first method of operation of the pixel layout of Figure 5.
  • Figure 3 shows a first pixel arrangement in accordance with the invention.
  • Each pixel comprises the current driven light emitting display element 2 in series with a drive transistor 22 between a power supply line 26 and ground 28.
  • the pixel storage capacitor 24 is connected between the data input point 30 to the pixel and the gate of the drive transistor 22.
  • the input to the pixel is supplied to the point 30 from the column data line 32 through the input, address, transistor 16.
  • a discharge transistor 34 is connected between the gate and drain of the drive transistor 22, and this is used to discharge the voltage on the gate of the drive transistor during a threshold sampling operation.
  • a coupling transistor 36 is connected between the data input 30 to the pixel and the power supply line 26, and this is used to couple the power supply line voltage to one terminal of the capacitor in order to provide a capacitive coupling effect, as will become apparent further below.
  • the pixel is operable to store a voltage on the storage capacitor 24 which is dependent both on an input voltage supplied to the data input and on the drive transistor threshold voltage.
  • the threshold voltage component is obtained by discharging the capacitor using the discharge transistor 34. During this time, the voltage at the data input 30 is much higher than the desired pixel data voltage.
  • the drive transistor can be turned off during the threshold sampling function without loss of the pixel data information, which is still held at the data input point 30.
  • the capacitive coupling using the coupling transistor 36 subsequently lowers the voltage at the point 30 in order to provide the desired voltage on the gate of the drive transistor 22, without the need to sample another signal from the data line 32.
  • the input transistor 16 and the coupling transistor 36 are controlled in complementary manner, by a shared control line A- ⁇ .
  • they are of different polarity types, the address transistor being n-type and the coupling transistor being p-type in the example shown. They may, of course, have their own control lines and may be of the same polarity type if desired.
  • the source of the drive transistor is connected to the power supply line 26, and the drain of the drive transistor is connected to the LED element 2 through a third transistor 38. This enables the voltage on the drain of the drive transistor to be isolated from the display element 2 during the threshold sampling operation.
  • the drive transistor 22 may comprise an LTPS p-type transistor, although the invention can also be used for amorphous silicon circuits, with n- type transistors only.
  • the circuit operation is to drive the p-type drive transistor by pulling the gate low, and providing a large voltage across the storage capacitor, but which is a function of the desired pixel drive level.
  • the drive transistor is then discharged, and the large capacitor voltage allows this to take place.
  • the voltage on the other side of the capacitor is then stepped. This voltage on the other side is then a function of the pixel drive voltage, and the step change in voltage results in a step change in the gate voltage so that the transistor turns on again, with the gate voltage dependent both on the pixel drive voltage and the threshold voltage.
  • the circuit operation comprises the following steps, shown in Figure 4.
  • plots are shown for the three control lines Ai to A3 which control transistors 16 and 36 (A-i), 34 (A2) and 38 (A3).
  • Figure 4 also shows as hatched the time when the input voltage is provided to the pixel.
  • the transistor 38 In order to drive current through the display element, the transistor 38 needs to be turned on, and it is on all of the time other than during the addressing cycle.
  • control line A3 goes low.
  • Control lines A 2 and A 3 then also go high. This has the effect of pulling the gate voltage of the drive transistor 22 to the anode voltage of the display element, as both transistors 34 and 38 are on. It is not important what this anode voltage is, providing it is low enough for the drive transistor to drive current through the display element. There is a brief pulse of light during this phase. The voltage across the capacitor is then Vp ⁇ + Vsup - V A NO DE . The control line A3 then goes low, and the pulse of light ceases. The drain of the drive transistor is then floating, but the gate-source voltage of the drive transistor causes it to continue driving current. This current acts to discharge the capacitor 24.
  • This capacitor discharge causes the gate voltage to rise (as the input node of the capacitor is still fixed at Vp ⁇ + V S U P )-
  • the drive transistor switches off when the gate voltage reaches V S UP - V THR ESH, where VTHRESH is the threshold voltage of the drive transistor 22.
  • Ai and A 2 go low, and this isolates the column data line 32 from the pixel and also connects the input side of the capacitor to the power supply line 26 through the transistor 36.
  • the voltage across the capacitor remains unchanged, and the step change in voltage on the input side of the capacitor is capacitively coupled to the gate of the drive transistor 22.
  • the gate-source voltage is thus Vp ⁇ + V T H RES H.
  • a 3 goes high at the end of addressing to drive the display element.
  • the pixel design of the invention avoids the need for two separate storage capacitors, reducing the pixel complexity and increasing the pixel aperture.
  • the pixel layout also reduces the effects of leakage currents, as the input point 30 is actively held at the desired voltage during the addressing cycle.
  • the data column does not need to carry a specially adapted waveform, but simply carries drive levels in conventional manner.
  • the voltage at the gate node side of the capacitor is:
  • V S U P - REAL is the real power line voltage
  • VSU P - IDEA L is the ideal voltage level.
  • the power lines Ai and A 3 may all be combined. This means the middle pulse in A3 shown in Figure 4 will not be present. This enables a reduction in the number of control lines.
  • A2 is arranged to pulse on slightly before A1 so that the gate is pulled low before the threshold measurement takes place.
  • the timing is shown in Figure 6.
  • the addressing starts with the pulse on A2, which causes a brief flash of light and ensures the drive transistor is driving current.
  • the pulse A1 begins, the pixel data is loaded onto one side of the capacitor and the threshold sampling takes place simultaneously on the other side of the capacitor in the manner explained above.
  • the circuit of the invention can be used for currently available LED devices.
  • the electroluminescent (EL) display element may comprise an electrophosphorescent organic electroluminescent display element.
  • the invention can be applied to LTPS or a-Si:H active matrix OLED displays.
  • the power supply line voltage may be of the order of 10 Volts
  • the over-threshold pixel data voltage may be in the approximate range 2-5 Volts, so that the voltage to be provided to the column will be in the range 12-15 Volts in this example.
  • the invention provides a voltage-addressed pixel which compensates for the effect of threshold voltage variations on the voltage-current conversion operation of the pixel.
  • the invention provides a circuit which enables a threshold voltage and a data voltage to be combined on one capacitor.
  • the circuit enables the drive transistor to be driven using charge from the first capacitor, until the drive transistor turns off, at which point the first capacitor stores a voltage derived from the threshold gate-source voltage and the data voltage, but with a voltage shift. In order to turn the transistor on again, this voltage shift is removed by a capacitive coupling effect.

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  • Physics & Mathematics (AREA)
  • Computer Hardware Design (AREA)
  • General Physics & Mathematics (AREA)
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Abstract

Each pixel of an active matrix display has a current driven light emitting display element (2), a drive transistor (22; TD) for driving a current through the display element from a power supply line (26) and a storage capacitor (24) connected between a data input to the pixel and the gate of the drive transistor. A discharge transistor (34) is connected between the gate and drain of the drive transistor and a coupling transistor (36) is connected between the data input to the pixel and the power supply line. The pixel is operable to store a voltage on the storage capacitor which is dependent both on an input voltage supplied to the data input and on the drive transistor threshold voltage. This arrangement enables a single capacitor to store a voltage derived both from the data input and the threshold voltage, and without needing to provide a complicated waveform to the data conductor.

Description

DESCRIPTION
ACTIVE MATRIX DISPLAY DEVICES
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.
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 IN-V semiconductor compounds. Recent developments in organic electroluminescent materials, 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.
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 exhibit diode-like I-V properties, so that they are capable of providing both a display function and a switching function, and can therefore 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-driven display elements, so that a conventional, analogue drive scheme involves supplying a 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 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.
Figure 2 shows in simplified schematic form a known pixel and drive circuitry arrangement for providing voltage-programmed 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 draws a current from the power supply line 26. The majority of active matrix circuits for LED displays have used low temperature polysilicon (LTPS) TFTs. The threshold voltage of these devices is stable in time, but varies from pixel to pixel in a random manner. This leads to unacceptable static noise in the image. Many circuits have been proposed to overcome this problem. In one example, each time the pixel is addressed the pixel circuit measures the threshold voltage of the current-providing TFT to overcome the pixel-to-pixel variations. The use of a-Si:H is now also being considered. 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. There will therefore be a large difference in the threshold voltage of an amorphous silicon transistor that is always on compared with one that is not. This differential ageing is a serious problem in LED displays driven with amorphous silicon transistors. The article "A new pixel circuit for active matrix organic light emitting diodes" in IEEE Electron Device Letters, IEEE Inc., New York, US, vol. 23 no. 9, September 2002, pages 514-546 discloses a threshold compensation circuit specifically for LTPS pixel circuits. WO-2004/066249 discloses a threshold compensation circuit specifically for amorphous silicon pixel circuits. In each case, each pixel has first and second capacitors connected in series between the gate and source or drain of the drive transistor. The drive transistor threshold voltage is stored on the first capacitor, and the second capacitor is charged to the pixel data voltage. This pixel arrangement enables a threshold voltage to be stored each time the pixel is addressed. A disadvantage of this pixel arrangement is the need for two relatively large capacitors, and the circuit also requires three address lines, to control the various phases of operation of the pixel circuit.
There remains a need for a threshold voltage compensation pixel design and addressing scheme with minimum additional pixel circuit complexity. According to the invention, there is provided an active matrix device comprising an array of display pixels, each pixel comprising: a current driven light emitting display element; a drive transistor for driving a current through the display element from a power supply line; a storage capacitor connected between a data input to the pixel and the gate of the drive transistor; a discharge transistor connected between the gate and drain of the drive transistor; a coupling transistor connected between the data input to the pixel and the power supply line, wherein the pixel is operable to store a voltage on the storage capacitor which is dependent both on an input voltage supplied to the data input and on the drive transistor threshold voltage. This arrangement uses a single capacitor to store a voltage derived both from the data input and the threshold voltage. The threshold voltage is sampled by discharging the capacitor using a discharge transistor connected between the gate and drain. This sampling operation results in the drive transistor turning off. In order to provide the required voltage on the gate of the drive transistor to turn it on again, capacitive coupling is used through the capacitor, and a coupling transistor is used to change the voltage at the data input to the pixel. This change is then coupled through the capacitor to the gate so give the desired drive transistor gate voltage.
Each pixel preferably further comprises an input first transistor connected between a data line and the data input to the pixel. The input first transistor and the coupling transistor can be controlled in complementary manner, by a shared control line.
The source of the drive transistor may be connected to the power supply line. Preferably, each pixel further comprises a third transistor connected between the drain of the drive transistor and the display element. This enables the voltage on the drain of the drive transistor to be isolated from the display element when the threshold sampling is taking place. This third transistor can be controlled by the shared control line, so that the circuit can be implemented with only two control lines.
The drive transistor preferably comprises an LTPS p-type transistor, although the invention can also be used for amorphous silicon circuits.
The display element preferably comprises an electroluminescent (EL) display element.
The invention also provides a method of driving an active matrix display device comprising an array of current driven light emitting display pixels, each pixel comprising an display element and a drive transistor for driving a current through the display element from a power supply line, the method comprising, for each pixel: applying an input voltage level derived from a desired pixel data voltage to one terminal of a storage capacitor, the other terminal of the storage capacitor being connected to the gate of the drive transistor; turning on the drive transistor; discharging the storage capacitor using the drive transistor until the drive transistor turns off, the one terminal of the first capacitor then carrying the input voltage level and the other terminal of the storage capacitor then carrying a voltage dependent on the threshold voltage; and isolating the input voltage from the one terminal of the storage capacitor and coupling the one terminal of the storage capacitor to the power supply line.
This method enables a single capacitor to be used to store a voltage combining a desired pixel drive voltage and a threshold voltage. When coupling the one terminal of the storage capacitor to the power supply line, capacitive coupling of the change in voltage on the one terminal preferably causes the other terminal of the capacitor to carry a voltage which differs from the power supply line by the summation of the threshold voltage and the desired pixel data voltage. The input voltage level is preferably greater than the power supply voltage, for example greater than the power supply voltage by the desired pixel data voltage
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 schematic diagram of a known pixel circuit for current- addressing the EL display pixel using an input drive voltage; Figure 3 shows a schematic diagram of a first example of pixel layout for a display device of the invention;
Figure 4 is a timing diagram for a first method of operation of the pixel layout of Figure 3;
Figure 5 shows a schematic diagram of a second example of pixel layout for a display device of the invention; and
Figure 6 is a timing diagram for a first method of operation of the pixel layout of Figure 5.
The same reference numerals are used in different figures for the same components, and description of these components will not be repeated.
Figure 3 shows a first pixel arrangement in accordance with the invention.
Each pixel comprises the current driven light emitting display element 2 in series with a drive transistor 22 between a power supply line 26 and ground 28. The pixel storage capacitor 24 is connected between the data input point 30 to the pixel and the gate of the drive transistor 22. The input to the pixel is supplied to the point 30 from the column data line 32 through the input, address, transistor 16.
A discharge transistor 34 is connected between the gate and drain of the drive transistor 22, and this is used to discharge the voltage on the gate of the drive transistor during a threshold sampling operation. A coupling transistor 36 is connected between the data input 30 to the pixel and the power supply line 26, and this is used to couple the power supply line voltage to one terminal of the capacitor in order to provide a capacitive coupling effect, as will become apparent further below. The pixel is operable to store a voltage on the storage capacitor 24 which is dependent both on an input voltage supplied to the data input and on the drive transistor threshold voltage. The threshold voltage component is obtained by discharging the capacitor using the discharge transistor 34. During this time, the voltage at the data input 30 is much higher than the desired pixel data voltage. Thus, even though the pixel data voltage has already been provided to the pixel, the drive transistor can be turned off during the threshold sampling function without loss of the pixel data information, which is still held at the data input point 30. The capacitive coupling using the coupling transistor 36 subsequently lowers the voltage at the point 30 in order to provide the desired voltage on the gate of the drive transistor 22, without the need to sample another signal from the data line 32.
As shown, the input transistor 16 and the coupling transistor 36 are controlled in complementary manner, by a shared control line A-ι. To provide the complementary operation, they are of different polarity types, the address transistor being n-type and the coupling transistor being p-type in the example shown. They may, of course, have their own control lines and may be of the same polarity type if desired.
The source of the drive transistor is connected to the power supply line 26, and the drain of the drive transistor is connected to the LED element 2 through a third transistor 38. This enables the voltage on the drain of the drive transistor to be isolated from the display element 2 during the threshold sampling operation.
The drive transistor 22 may comprise an LTPS p-type transistor, although the invention can also be used for amorphous silicon circuits, with n- type transistors only.
The circuit operation is to drive the p-type drive transistor by pulling the gate low, and providing a large voltage across the storage capacitor, but which is a function of the desired pixel drive level. The drive transistor is then discharged, and the large capacitor voltage allows this to take place. This places a voltage on one side of the capacitor which is a function of the threshold voltage. The voltage on the other side of the capacitor is then stepped. This voltage on the other side is then a function of the pixel drive voltage, and the step change in voltage results in a step change in the gate voltage so that the transistor turns on again, with the gate voltage dependent both on the pixel drive voltage and the threshold voltage.
The circuit operation comprises the following steps, shown in Figure 4. In Figure 4, plots are shown for the three control lines Ai to A3 which control transistors 16 and 36 (A-i), 34 (A2) and 38 (A3). Figure 4 also shows as hatched the time when the input voltage is provided to the pixel.
In order to drive current through the display element, the transistor 38 needs to be turned on, and it is on all of the time other than during the addressing cycle.
At the beginning of the addressing cycle, the transistor 38 is turned off, and control line A3 goes low.
During a first phase of the addressing cycle, only address line Ai is high, and allows a voltage from the column data line 32 to pass to the point 30. The voltage provided on the column data line is the summation of the desired over-threshold pixel drive voltage VPιχ and the power supply line voltage VSUP.- With the transistor 36 turned off by A-i, the input terminal of the capacitor (i.e. point 30) is thus charged to VP|X + VSUP-
Control lines A2 and A3 then also go high. This has the effect of pulling the gate voltage of the drive transistor 22 to the anode voltage of the display element, as both transistors 34 and 38 are on. It is not important what this anode voltage is, providing it is low enough for the drive transistor to drive current through the display element. There is a brief pulse of light during this phase. The voltage across the capacitor is then Vpιχ + Vsup - VANODE. The control line A3 then goes low, and the pulse of light ceases. The drain of the drive transistor is then floating, but the gate-source voltage of the drive transistor causes it to continue driving current. This current acts to discharge the capacitor 24.
This capacitor discharge causes the gate voltage to rise (as the input node of the capacitor is still fixed at Vpιχ + VSUP)- The drive transistor switches off when the gate voltage reaches VSUP - VTHRESH, where VTHRESH is the threshold voltage of the drive transistor 22.
The voltage across the capacitor is then (VP|X + VSup) - (VSUP - VTHRESH), namely VP|X + VTHRESH.
In the third phase, Ai and A2 go low, and this isolates the column data line 32 from the pixel and also connects the input side of the capacitor to the power supply line 26 through the transistor 36. The voltage across the capacitor remains unchanged, and the step change in voltage on the input side of the capacitor is capacitively coupled to the gate of the drive transistor 22. The gate-source voltage is thus Vpιχ + VTHRESH. A3 goes high at the end of addressing to drive the display element.
The pixel design of the invention avoids the need for two separate storage capacitors, reducing the pixel complexity and increasing the pixel aperture. The pixel layout also reduces the effects of leakage currents, as the input point 30 is actively held at the desired voltage during the addressing cycle. The data column does not need to carry a specially adapted waveform, but simply carries drive levels in conventional manner.
The effects of power line cross talk are also reduced. If the local power line voltage drops, the voltage on the gate at which the drive transistor turns off, during the threshold voltage sampling operation, will be lower. In particular, the gate voltage will be lower by the power line voltage drop. The voltage at the input side of the capacitor (point 30) is then:
(Vp|χ + VsUP-IDEAL.)
The voltage at the gate node side of the capacitor is:
(VSUP-REAL - VTHRESH) The voltage across the capacitor is thus:
Vpιχ + VTHRESH. + (VSUP-IDEAL - VSUP-REAL)- In these equations, VSUP-REAL is the real power line voltage and VSUP-IDEAL is the ideal voltage level.
In this way, an increased gate-source voltage is provided in response to a reduced power line voltage, and this provides a form of compensation for the power line voltage drops.
As shown in Figure 5, the power lines Ai and A3 may all be combined. This means the middle pulse in A3 shown in Figure 4 will not be present. This enables a reduction in the number of control lines.
In this case, A2 is arranged to pulse on slightly before A1 so that the gate is pulled low before the threshold measurement takes place. The timing is shown in Figure 6.
The addressing starts with the pulse on A2, which causes a brief flash of light and ensures the drive transistor is driving current. When the pulse A1 begins, the pixel data is loaded onto one side of the capacitor and the threshold sampling takes place simultaneously on the other side of the capacitor in the manner explained above.
The circuit of the invention can be used for currently available LED devices. However, the electroluminescent (EL) display element may comprise an electrophosphorescent organic electroluminescent display element. The invention can be applied to LTPS or a-Si:H active matrix OLED displays.
By way of example, the power supply line voltage may be of the order of 10 Volts, and the over-threshold pixel data voltage may be in the approximate range 2-5 Volts, so that the voltage to be provided to the column will be in the range 12-15 Volts in this example. The invention provides a voltage-addressed pixel which compensates for the effect of threshold voltage variations on the voltage-current conversion operation of the pixel.
There are other variations to the specific circuit layout which can work in the same way. Essentially, the invention provides a circuit which enables a threshold voltage and a data voltage to be combined on one capacitor. To store the threshold voltage, the circuit enables the drive transistor to be driven using charge from the first capacitor, until the drive transistor turns off, at which point the first capacitor stores a voltage derived from the threshold gate-source voltage and the data voltage, but with a voltage shift. In order to turn the transistor on again, this voltage shift is removed by a capacitive coupling effect. Various other modifications will be apparent to those skilled in the art.

Claims

1. An active matrix device comprising an array of display pixels, each pixel comprising: a current driven light emitting display element (2); a drive transistor (22;TD) for driving a current through the display element from a power supply line; a storage capacitor (24) connected between a data input (30) to the pixel and the gate of the drive transistor (22); a discharge transistor (34) connected between the gate and drain of the drive transistor; a coupling transistor (36) connected between the data input to the pixel and the power supply line, wherein the pixel is operable to store a voltage on the storage capacitor (24) which is dependent both on an input voltage supplied to the data input and on the drive transistor threshold voltage.
2. A device as claimed in claim 1 , wherein each pixel further comprises an input first transistor (16) connected between a data line (32) and the data input (30) to the pixel.
3. A device as claimed in claim 2, wherein the input first transistor (16) and the coupling transistor are controlled in complementary manner.
4. A device as claimed in claim 3, wherein the input first transistor (16) and the coupling transistor (36) are of opposite polarity type and are controlled by a shared control line (A-i).
5. A device as claimed in any preceding claim, wherein the source of the drive transistor (22;TD) is connected to the power supply line (26).
6. A device as claimed in any preceding claim, wherein each pixel further comprises a third transistor (38) connected between the drain of the drive transistor (22) and the display element (2).
7. A device as claimed in claim 6 and claim 4, wherein the third transistor (38) is controlled by the shared control line (Ai), and is of the same polarity type as the coupling transistor (36).
8. A device as claimed in any preceding claim, wherein the drive transistor (22;Tp) comprises an LTPS p-type transistor.
9. A device as claimed in any preceding claim, wherein the display element (2) comprises an electroluminescent (EL) display element.
10. A method of driving an active matrix display device comprising an array of current driven light emitting display pixels, each pixel comprising an display element (2) and a drive transistor (22;TD) for driving a current through the display element (2) from a power supply line (26), the method comprising, for each pixel: applying an input voltage level derived from a desired pixel data voltage to one terminal of a storage capacitor (24), the other terminal of the storage capacitor being connected to the gate of the drive transistor (22); turning on the drive transistor; discharging the storage capacitor (24) using the drive transistor (22) until the drive transistor turns off, the one terminal (30) of the first capacitor then carrying the input voltage level and the other terminal of the storage capacitor then carrying a voltage dependent on the threshold voltage; and isolating the input voltage from the one terminal of the storage capacitor and coupling the one terminal of the storage capacitor to the power supply line.
11. A method as claimed in claim 10, wherein when coupling the one terminal of the storage capacitor to the power supply line, capacitive coupling of the change in voltage on the one terminal (30) causes the other terminal of the capacitor to carry a voltage which differs by the summation of the threshold voltage and the desired pixel data voltage.
12. A method as claimed in claim 10 or 11 , wherein the input voltage level is greater than the power supply voltage.
13. A method as claimed in claim 12, wherein the input voltage level is greater than the power supply voltage by the desired pixel data voltage
PCT/IB2005/052511 2004-07-29 2005-07-26 Active matrix display devices WO2006013539A1 (en)

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Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN103839513A (en) * 2012-11-27 2014-06-04 乐金显示有限公司 Organic light emitting diode display device and method of driving the same
EP2863380A1 (en) * 2013-10-21 2015-04-22 EverDisplay Optronics (Shanghai) Limited Pixel driving circuit and display device
TWI603309B (en) * 2013-02-25 2017-10-21 三星顯示器有限公司 Pixel, display device including the same, and method thereof

Citations (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1533782A2 (en) * 2003-11-24 2005-05-25 Samsung SDI Co., Ltd. Light emitting display and driving method thereof

Patent Citations (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1533782A2 (en) * 2003-11-24 2005-05-25 Samsung SDI Co., Ltd. Light emitting display and driving method thereof

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
CHOI S-M ET AL: "AN IMPROVED VOLTAGE PROGRAMMED PIXEL STRUCTURE FOR LARGE SIZE AND HIGH RESOLUTION AM-OLED DISPLAYS", 25 May 2004, 2004 SID INTERNATIONAL SYMPOSIUM DIGEST OF TECHNICAL PAPERS. SEATTLE, WA, MAY 25 - 27, 2004, SID INTERNATIONAL SYMPOSIUM DIGEST OF TECHNICAL PAPERS, SAN JOSE, CA : SID, US, PAGE(S) 260-263, XP001222795 *

Cited By (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN103839513A (en) * 2012-11-27 2014-06-04 乐金显示有限公司 Organic light emitting diode display device and method of driving the same
EP2736036A3 (en) * 2012-11-27 2014-06-18 LG Display Co., Ltd. Organic light emitting diode display device and method of driving the same
US9111488B2 (en) 2012-11-27 2015-08-18 Lg Display Co., Ltd. Organic light emitting diode display device and method of driving the same
CN103839513B (en) * 2012-11-27 2016-03-09 乐金显示有限公司 Organic light-emitting diode (OLED) display apparatus and driving method thereof
TWI603309B (en) * 2013-02-25 2017-10-21 三星顯示器有限公司 Pixel, display device including the same, and method thereof
US10885844B2 (en) 2013-02-25 2021-01-05 Samsung Display Co., Ltd. Pixel, display device including the same and method thereof
EP2863380A1 (en) * 2013-10-21 2015-04-22 EverDisplay Optronics (Shanghai) Limited Pixel driving circuit and display device
TWI514351B (en) * 2013-10-21 2015-12-21 Everdisplay Optronics Shanghai Ltd Pixel driving circuit and display device

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