Reference is made to commonly-assigned, co-pending U.S. patent application Ser. No. 11/766,823, filed Jun. 22, 2007, entitled “OLED Display with Aging and Efficiency Compensations” by Levey et al, and to commonly-assigned, co-pending U.S. patent application Ser. No. 11/962,182, filed Dec. 21, 2007, entitled “Electroluminescent Display Compensated Analog Transistor Drive Signal” by Leon et al, the disclosures of which are incorporated by reference herein.
The present invention relates to solid-state electroluminescent (EL) flat-panel displays, such as organic light-emitting diode (OLED) displays, and more particularly to such displays having a way to compensate for the aging of the electroluminescent display components.
Electroluminescent (EL) devices have been known for some years and have been recently used in commercial display devices. Such devices employ both active-matrix and passive-matrix control schemes and can employ a plurality of subpixels. Each subpixel contains an EL emitter and a drive transistor for driving current through the EL emitter. The subpixels are typically arranged in two-dimensional arrays with a row and a column address for each subpixel, and having a data value associated with the subpixel. Subpixels of different colors, such as red, green, blue and white, are grouped to form pixels. EL displays can be made from various emitter technologies, including coatable-inorganic light-emitting diode, quantum-dot, and organic light-emitting diode (OLED).
OLED displays are of particular interest as a superior flat-panel display technology. These displays utilize current passing through thin films of organic material to generate light. The color of light emitted and the efficiency of the energy conversion from current to light are determined by the composition of the organic thin-film material. Different organic materials emit different colors of light. However, as the display is used, the organic materials in the display age and become less efficient at emitting light. This reduces the lifetime of the display. The differing organic materials can age at different rates, causing differential color aging and a display whose white point varies as the display is used. In addition, each individual pixel can age at a rate different from other pixels, resulting in display nonuniformity. Further, some circuitry elements, e.g. amorphous silicon transistors, are also known to exhibit aging effects.
The rate at which the materials age is related to the amount of current that passes through the display and, hence, the amount of light that has been emitted from the display. Various techniques to compensate for this aging effect have been described.
U.S. Pat. No. 6,414,661 B1 by Shen et al. describes a method and associated system to compensate for long-term variations in the light-emitting efficiency of individual organic light-emitting diodes (OLEDs) in an OLED display by calculating and predicting the decay in light output efficiency of each pixel based on the accumulated drive current applied to the pixel. The method derives a correction coefficient that is applied to the next drive current for each pixel. This technique requires the measurement and accumulation of drive current applied to each pixel, requiring a stored memory that must be continuously updated as the display is used, and therefore requiring complex and extensive circuitry.
U.S. Pat. No. 6,504,565 B1 by Narita et al. describes a similar method of holding the amount of light emitted from each light-emitting element constant. This design requires the use of a calculation unit responsive to each signal sent to each pixel to record usage, greatly increasing the complexity of the circuit design.
U.S. Patent Application Publication No. 2002/0167474 A1 by Everitt describes a pulse width modulation driver for an OLED display. One embodiment of a video display comprises a voltage driver for providing a selected voltage to drive an organic light-emitting diode in a video display. The voltage driver can receive voltage information from a correction table that accounts for aging, column resistance, row resistance, and other diode characteristics. In one embodiment of the invention, the correction tables are calculated prior to and/or during normal circuit operation. Since the OLED output light level is assumed to be linear with respect to OLED current, the correction scheme is based on sending a known current through the OLED diode for a duration sufficiently long to permit the transients to settle out, and then measuring the corresponding voltage with an analog-to-digital converter (A/D) residing on the column driver. A calibration current source and the A/D can be switched to any column through a switching matrix.
JP 2002-278514A by Numao describes a method in which current through and temperature of organic EL elements are measured. Compensation is then performed using precomputed tables and the current and temperature measurements. This design presumes a predictable relative use of pixels and does not accommodate differences in actual usage of groups of pixels or of individual pixels. Hence, correction for color or spatial groups is likely to be inaccurate over time. Moreover, the integration of temperature and multiple current sensing circuits within the display is required. This integration is complex, reduces manufacturing yields, and takes up space within the display.
U.S. Patent Application Publication No. 2003/0122813 A1 by Ishizuki et al. discloses a method which measures current for each subpixel in turn. The measurement techniques of this method are iterative, and therefore slow.
U.S. Pat. No. 6,995,519, by Arnold et al., teaches a method of compensating for aging of an OLED emitter. This method assumes that the entire change in device luminance is caused by changes in the OLED emitter. However, when the drive transistors in the circuit are formed from amorphous silicon (a-Si), this assumption is not valid, as the threshold voltage of the transistors also changes with use. This method will not provide complete compensation for OLED efficiency losses in circuits wherein transistors show aging effects. Additionally, when methods such as reverse bias are used to mitigate a-Si transistor threshold voltage shifts, compensation of OLED efficiency loss can become unreliable without appropriate tracking/prediction of reverse bias effects, or a direct measurement of the OLED voltage change or transistor threshold voltage change.
U.S. Patent Application Publication No. 2004/0100430 A1 by Fruehauf discloses a pixel structure having a third transistor which taps a diode driving current to supply a current-measuring circuit and a voltage comparison unit. However, this method reduces the efficiency of a display containing such pixels by using for measurement current which could have otherwise been used to emit light. Furthermore, this method only compensates for TFT variations and is unable to compensate for non-uniform OLED characteristics.
In addition to aging effects, some transistor technologies, such as low-temperature polysilicon (LTPS), can produce drive transistors that have varying mobilities and threshold voltages across the surface of a display (Kuo, Yue, ed. Thin Film Transistors: Materials and Processes, vol. 2: Polycrystalline Thin Film Transistors. Boston: Kluwer Academic Publishers, 2004, pg. 410-412). This produces objectionable visible nonuniformity. Further, nonuniform OLED material deposition can produce emitters with varying efficiencies, also causing objectionable nonuniformity. These nonuniformities are present at the time the panel is sold to an end user, and so are termed initial nonuniformities.
U.S. Pat. No. 6,081,073 by Salam describes a display matrix with a process and control circuitry for reducing brightness variations in the pixels. This disclosure describes the use of a linear scaling method for each pixel based on a ratio between the brightness of the weakest pixel in the display and the brightness of each pixel. However, this approach will lead to an overall reduction in the dynamic range and brightness of the display and a reduction and variation in the bit depth at which the pixels can be operated.
U.S. Pat. No. 6,473,065 B1 by Fan describes methods of improving the display uniformity of an OLED. The display characteristics of all organic-light-emitting-elements are measured. The technique uses a combination of look-up tables and calculation circuitry to implement uniformity correction. However, this method requires optical measurements. This makes it unsuitable for aging correction, which requires periodic measurement in the user's location. Further, the described approaches require either a separate lookup table for each pixel, resulting in very expensive memory requirements, or approximations to the characteristics of each pixel, reducing image quality.
U.S. Patent Application Publication No. 2005/0007392 A1 by Kasai et al. describes an electro-optical device that stabilizes display quality by performing correction processing corresponding to a plurality of disturbance factors, and using a conversion table whose description contents include correction factors. However, this method requires a large number of look-up tables (LUTs), not all of which are in use at any given time, to perform processing, and does not describe a method for populating those LUTs.
There is a need therefore for a more complete compensation approach for aging and initial nonuniformity of electroluminescent displays.
It is therefore an object of the present invention to compensate for aging and efficiency changes in electroluminescent emitters in the presence of transistor aging.
This object is achieved by a method of providing drive transistor control signals to drive transistors in a plurality of electroluminescent (EL) subpixels, comprising:
(a) providing a plurality of EL subpixels, each subpixel including a drive transistor having a first electrode, a second electrode and a gate electrode, an EL emitter having a first electrode and a second electrode, and a readout transistor having a first electrode, a second electrode and a gate electrode;
(b) connecting the first electrode of each readout transistor to the second electrode of the corresponding drive transistor and to the first electrode of the corresponding EL emitter;
(c) receiving for each subpixel an input code value which commands a corresponding output from the respective subpixel,
(d) selecting a target subpixel;
(e) providing to each subpixel, except the target subpixel, the respective input code value, and providing to the target subpixel a boosted code value which commands a selected first amount higher output than the corresponding input code value;
(f) after a selected delay time, measuring a readout voltage on the second electrode of the readout transistor of the target subpixel to provide a status signal representing the characteristics of the drive transistor and EL emitter in that subpixel;
(g) using the status signal to provide a compensated code value for the target subpixel;
(h) providing a drive transistor control signal corresponding to the compensated code value to the drive transistor of the target EL subpixel; and
(i) repeating steps (d) through (h), selecting each of the plurality of subpixels in turn as the target subpixel, to provide a respective drive transistor control signal to the drive transistor in each of the plurality of EL subpixels.
This aim is further achieved by an apparatus for providing a drive transistor control signal to the gate electrode of a drive transistor in an electroluminescent (EL) subpixel, comprising:
a) the EL subpixel including the drive transistor having first, second and gate electrodes, an EL emitter having first and second electrodes, and a readout transistor having a first electrode connected to the second electrode of the drive transistor and having a second electrode, wherein the first electrode of the EL emitter is connected to the second electrode of the drive transistor;
b) a measurement circuit for measuring a readout voltage on the second electrode of the readout transistor at different times to provide a status signal representing variations in the characteristics of the drive transistor and EL emitter caused by operation of the drive transistor and EL emitter over time;
c) means for providing an input code value;
d) a compensator for receiving an input code value and producing a compensated code value in response to the status signal; and
e) a source driver for producing the drive transistor control signal in response to the compensated code value for driving the gate electrode of the drive transistor.
An advantage of this invention is an OLED display that compensates for the aging of the organic materials in the display wherein circuitry aging is also occurring, without requiring extensive or complex circuitry for accumulating a continuous measurement of light-emitting element use or time of operation. It is a further advantage of this invention that it uses simple voltage measurement circuitry. It is a further advantage of this invention that by making all measurements of voltage, it is more sensitive to changes than methods that measure current. It is a further advantage of this invention that compensation for changes in driving transistor properties can be performed with compensation for the OLED changes, thus providing a complete compensation solution. It is a further advantage of this invention that both aspects of measurement and compensation (OLED and driving transistor) can be accomplished rapidly. It is a further advantage of this invention that a single select line can be used to enable data input and data readout. It is a further advantage of this invention that characterization and compensation of driving transistor and OLED changes are unique to the specific element and are not impacted by other elements that may be open-circuited or short-circuited.
Turning to
Turning now to
The first electrode of readout transistor 80 is connected to the second electrode of drive transistor 70 and also to the first electrode of EL emitter 50. Each readout line 30 is connected to the second electrodes of the readout transistors 80 in the corresponding column of EL subpixels 60. Readout line 30 provides a readout voltage to measurement circuit 170, which measures the readout voltage to provide status signals representative of characteristics of EL subpixel 60.
A plurality of readout lines 30 can be connected to measurement circuit 170 through a multiplexer-output line 45 and multiplexer 40 for sequentially reading out the voltages from the second electrodes of the respective readout transistors of a predetermined number of EL subpixels 60. If there are a plurality of multiplexers 40, each can have its own multiplexer-output line 45. Thus, a predetermined number of EL subpixels can be driven simultaneously. The plurality of multiplexers will permit parallel reading out of the voltages from the various multiplexers 40, while each multiplexer would permit sequential reading out of the readout lines 30 attached to it. This will be referred to herein as a parallel/sequential process.
Measurement circuit 170 includes a conversion circuit 171 and optionally a processor 190 and a memory 195. Conversion circuit 171 receives a readout voltage on multiplexer-output line 45 and outputs digital data on a converted-data line 93. Conversion circuit 171 preferably presents a high input impedance to multiplexer-output line 45. The readout voltage measured by conversion circuit 171 can be equal to the voltage on the second electrode of readout transistor 90, or can be a function of that voltage. For example, the readout voltage measurement can be the voltage on the second electrode of readout transistor 90, minus the drain-source voltage of the readout transistor and the voltage drop across the multiplexer 40. The digital data can be used as a status signal, or the status signal can be computed by processor 190 as will be described below. The status signal represents the characteristics of the drive transistor and EL emitter in the EL subpixel 60. Processor 190 receives digital data on converted-data line 93 and outputs the status signal on a status line 94. Processor 190 can be a CPU, FPGA or ASIC, and can optionally be connected to memory 195. Memory 195 can be non-volatile storage such as Flash or EEPROM, or volatile storage such as SRAM.
A compensator 191 receives the status signal on status line 94 and an input code value on an input line 85, and provides a compensated code value on a control line 95. A source driver 155 receives the compensated code value and produces a drive transistor control signal on data line 35. Thus, processor 190 can provide compensated data as will be described herein during the display process. As known in the art, the input code value can be provided by a timing controller (not shown). The input code value can be digital or analog, and can be linear or nonlinear with respect to commanded luminance. If analog, the input code value can be a voltage, a current, or a pulse-width modulated waveform.
Source driver 155 can includes a digital-to-analog converter or programmable voltage source, a programmable current source, or a pulse-width modulated voltage (“digital drive”) or current driver, or another type of source driver known in the art.
Processor 190 and compensator 191 can be implemented on the same CPU or other hardware. Processor 190 and compensator 191 can together provide predetermined data values to data line 35 during the measurement process to be described herein.
Referring to
Referring to
While measurements are being taken, test data values can command the emission of light from the EL emitter. This can be undesirably visible to a user of the EL display. Drive transistors 70, as known in the art, have a threshold voltage Vth below which (or, for P-channel, above which) relatively little current flows, and so relatively little light is emitted. The selected reference voltage level can be less than the threshold voltage to prevent user-visible light from being emitted during measurement.
When drive transistor 70 is an amorphous silicon transistor, the threshold voltage Vth is known to change under aging conditions, including actual usage conditions. Driving current through EL emitter 50 thus leads to an increase in Vth of drive transistor 70. Therefore, a constant signal on the gate electrode of drive transistor 70 will cause a gradually decreasing current Ids, and thus a gradually decreasing light intensity emitted by EL emitter 50. The amount of such decrease will depend upon the use of drive transistor 70; thus, the decrease can be different for different drive transistors in a display. This is one type of spatial variation in characteristics of EL subpixels 60. Such spatial variation can include differences in brightness and color balance in different parts of the display, and image “burn-in” wherein an often-displayed image (e.g. a network logo) can cause a ghost of itself to always show on the active display. It is desirable to compensate for such changes in the threshold voltage to prevent such problems. Also, there can be age-related changes to EL emitter 50, e.g. luminance efficiency loss and an increase in resistance across EL emitter 50.
Turning now to
A further type of spatial variation is initial nonuniformity. The operating life of an EL display is the time from when an end user first sees an image on that display to the time when that display is discarded. Initial nonuniformity is any nonuniformity present at the beginning of the operating life of a display. The present invention can advantageously correct for initial nonuniformity by taking measurements before the operating life of the EL display begins. Measurements can be taken in the factory as part of production of a display. Measurements can also be taken after the user first activates a product containing an EL display, immediately before showing the first image on that display. This permits the display to present a high-quality image to the end user when he first sees it, so that his first impression of the display will be favorable.
Turning to
The relationship between the OLED current IEL (which is also the drain-source current Vds through the drive transistor), OLED voltage VEL, and threshold voltage at saturation Vth is:
where W is the TFT Channel Width, L is the TFT Channel Length, μ is the TFT mobility, C0 is the Oxide Capacitance per Unit Area, Vg is the gate voltage, and Vgs is voltage difference between gate and source of the drive transistor. For simplicity, we neglect dependence of μ on Vgs. Thus, to compensate for variations in characteristics of one or a plurality of EL subpixels 60, one must correct for change in Vth and VEL. However, taking multiple measurements can be very time-consuming. The present invention advantageously reduces measurement time by correcting for transistor and EL emitter variations with one measurement.
Referring to
For each subpixel, compensator 191 receives a corresponding input code value on input line 85 which commands a corresponding light output from the respective subpixel. Shown on the timing diagram of
Referring to
As shown on
The boosted code value period 302 prevents measurements from being visible by equalizing the light output of the target subpixel and the other subpixels. During the boosted code value period, the target subpixel can be driven at a higher output level to balance the shorter time it is on. Delay time 303 can be a selected percentage of a selected row time 307. The selected first amount is then a percentage of the output commanded by the corresponding input code value, and can be calculated as the reciprocal of the selected percentage. For example, if the delay time 303 is 0.8 (4/5) of row time 307, the selected first amount is 1/0.8=5/4=1.25. A 20% reduction in time available requires a 25% increase in luminance to produce the same total light output (100% output for one row time=1*1=1; 125% output for 0.8 row times=1.25*0.8=1).
Referring to
Turning now to
The status signal can represent aging: variations in the characteristics of the drive transistor 70 and EL emitter 50 in the target subpixel 60 caused by operation of the drive transistor and EL emitter in that subpixel over time. To calculate such a status signal, in either embodiment of conversion circuit 171 described above, a first readout voltage measurement can be taken of each subpixel and stored in memory 195 by processor 190. This measurement can be taken before the operating life of the EL display. During operation of the EL display, at a different, later time than the time at which the first readout voltage measurement was taken, a second readout voltage measurement can be taken of each subpixel and stored in memory 195. The first and second readout voltage measurements can then be used to compute a status signal representing variations in the characteristics of the drive transistor and EL emitter caused by operation of the drive transistor and EL emitter over time. For example, the status signal can then be calculated as the difference between the second readout voltage measurement and the first readout voltage measurement, or as a function of that difference, such as a linear transform.
The status signal is then provided to compensator 191, which provides a compensated code value for the target subpixel using the status signal and the input code value (Step 360). The operation of the compensator will be discussed further below.
A drive transistor control signal corresponding to the compensated code value is then provided to the drive transistor of the target EL subpixel. The compensator provides the compensated code value to source driver 155, which produces the drive transistor control signal and provides it via data line 35 and select transistor 80 to the gate electrode of drive transistor 70 (step 370).
Steps 320 through 370 are then repeated (decision step 380) until each of the plurality of subpixels in turn has been selected as the target subpixel and respective drive transistor control signals have been provided to the respective drive transistors in each of the plurality of EL subpixels. Once the readout voltage has been measured for a subpixel, the corresponding status signal can be stored in memory 195. The compensator 191 can use that stored status signal to compensate any number of input code values. Measurements can be taken at regular intervals, each time the display is powered up or down, or at intervals determined by the usage of the display. Measurements can also be taken throughout the life of the display as the boosted code value 302 prevents the measurement period 304 from being visible to the user. Subpixels can be selected to be the target subpixel in any order. In one embodiment, they can be selected from top to bottom, according to the row scanning order of the display, and from left to right or right to left. In another embodiment, target subpixels can be selected at random positions in each row to prevent systematic bias due to factors such as temperature gradients.
Referring back to
VEL=(Vout+Vread)−CV (Eq. 2)
Variations in the characteristics of the drive transistors and EL devices in the EL subpixels are reflected in variations in the calculated VEL. VEL can thus be used as a status signal. Before mass-production of EL display 10, one or more representative devices can be characterized to produce an product model mapping VEL for each subpixel to the corresponding transistor (Vth, mobility) and EL device (resistance, efficiency) characteristics. More than one product model can be created. For example, different regions of the display can have different product models. The product model can be stored in a lookup table or used as an algorithm.
In one embodiment, particularly useful for initial-nonuniformity compensation, a reference status signal level can be selected. This level can be the mean, minimum or maximum of the status signals for all subpixels, or another function as will be obvious to those skilled in the art. The compensator can compare each subpixel's respective status signal to the reference status signal level to determine how much compensation to apply. This can be useful when compensating for initial nonuniformity, in which case a second readout voltage measurement is not available. The compensator can use the product model with the measured VEL values and the selected reference status signal level to produce the compensated code values.
In one embodiment for aging compensation according to the present invention, the difference ΔVEL between VEL at the second readout voltage measurement and VEL at the first readout voltage measurement is used as the status signal. Amorphous silicon TFT aging and OLED aging are both proportional to the integrated current passed through the devices over time, so a model can be made correlating ΔVEL with ΔVth of the transistors and compensation performed.
In the case of
An additional effect in aging compensation is OLED efficiency loss. An example of the relationship between luminance efficiency and ΔVEL for one device is shown in the graph in
To compensate for the changes or variations in characteristics of EL subpixel 60, one can use the status signals in an equation of the form:
Vcomp=Vdata+f1(ΔVEL)+f2(ΔVEL)+f3(ΔVEL, Vdata) (Eq. 3)
where Vcomp is a voltage corresponding to the compensated code value necessary to maintain the desired luminance of EL subpixel 60, Vdata is a voltage corresponding to the input code value, f1(ΔVEL) is a correction for the change in threshold voltage, f2(ΔVEL) is a correction for the change in EL resistance, and f3(ΔVEL, Vdata) is a correction for the change in EL efficiency. Function f3 will be described further below. Functions f1, f2 and f3 are components of the product model. Using this equation, compensator 191 can control EL emitter 60 to achieve constant luminance output and increased lifetime at a given luminance. Because this method provides a respective correction for each EL subpixel in EL display 10, it will compensate for spatial variations in the characteristics of the plurality of EL subpixels.
In a preferred embodiment, the invention is employed in a display that includes Organic Light Emitting Diodes (OLEDs) which are composed of small molecule or polymeric OLEDs as disclosed in but not limited to U.S. Pat. No. 4,769,292, by Tang et al., and U.S. Pat. No. 5,061,569, by VanSlyke et al. Many combinations and variations of organic light emitting displays can be used to fabricate such a display. Referring to
Transistors 70, 80 and 90 can be amorphous silicon (a-Si) transistors, low-temperature polysilicon (LTPS) transistors, zinc oxide transistors, or other transistor types known in the art. They can be N-channel, P-channel, or any combination. The OLED can be a non-inverted structure (as shown) or an inverted structure in which EL emitter 50 is connected between first voltage source 140 and drive transistor 70.
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.
Number | Name | Date | Kind |
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6081073 | Salam | Jun 2000 | A |
6414661 | Shen et al. | Jul 2002 | B1 |
6473065 | Fan | Oct 2002 | B1 |
6504565 | Narita et al. | Jan 2003 | B1 |
6995519 | Arnold et al. | Feb 2006 | B2 |
7928936 | Levey et al. | Apr 2011 | B2 |
7994810 | Hayashi | Aug 2011 | B2 |
8026873 | Leon et al. | Sep 2011 | B2 |
20020267474 | Everitt | Nov 2002 | |
20030122813 | Ishizuki et al. | Jul 2003 | A1 |
20040100430 | Fruehauf | May 2004 | A1 |
20050007392 | Kasai et al. | Jan 2005 | A1 |
20060158402 | Nathan et al. | Jul 2006 | A1 |
20070195020 | Nathan et al. | Aug 2007 | A1 |
20080111812 | Shirasaki et al. | May 2008 | A1 |
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20100225634 | Levey et al. | Sep 2010 | A1 |
Number | Date | Country |
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2002-278514 | Sep 2002 | JP |
WO 2005109389 | Nov 2005 | WO |
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Number | Date | Country | |
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20100123649 A1 | May 2010 | US |