Patent Grant
8299983
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, the disclosure of which is incorporated by reference herein.
The present invention relates to solid-state electroluminescent flat-panel displays and more particularly to such displays having ways to compensate for differences in the characteristics of the various components composing such displays.
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). However, such displays suffer from a variety of defects that limit the quality of the displays. In particular, OLED displays suffer from visible nonuniformities in the subpixels across a display. These nonuniformities can be attributed to both the EL emitters in the display and, for active-matrix displays, to variability in the thin-film transistors used to drive the EL emitters.
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. 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.
It is known in the prior art to measure the performance of each pixel in a display and then to correct for the performance of the pixel to provide a more uniform output across the display. U.S. Patent Application Publication No. 2003/0122813 A1 by Ishizuki et al. discloses a display panel driving device and driving method for providing high-quality images without irregular luminance. The light-emission drive current flowing is measured while each pixel successively and independently emits light. Then the luminance is corrected for each input pixel data based on the measured drive current values. According to another aspect, the drive voltage is adjusted such that one drive current value becomes equal to a predetermined reference current. In a further aspect, the current is measured while an off-set current, corresponding to a leak current of the display panel, is added to the current output from the drive voltage generator circuit, and the resultant current is supplied to each of the pixel portions. The measurement techniques are iterative, and therefore slow. Further, this technique is directed at compensation for aging, not for initial nonuniformity.
U.S. Pat. No. 6,081,073 entitled “Matrix Display with Matched Solid-State Pixels” by Salam, describes a display matrix with a process and control circuitry for reducing brightness variations in the pixels. This patent 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 entitled “Methods of improving display uniformity of organic light emitting displays by calibrating individual pixel” by Fan, describes methods of improving the display uniformity of an OLED. In order to improve the display uniformity of an OLED, the display characteristics of all organic-light-emitting-elements are measured, and calibration parameters for each organic-light-emitting-element are obtained from the measured display characteristics of the corresponding organic-light-emitting-element. The calibration parameters of each organic-light-emitting-element are stored in a calibration memory. The technique uses a combination of look-up tables and calculation circuitry to implement uniformity correction. However, the described approaches require either a lookup table providing a complete characterization for each pixel, or extensive computational circuitry within a device controller. This is likely to be expensive and impractical in most applications.
U.S. Pat. No. 6,414,661 B1 entitled “Method and apparatus for calibrating display devices and automatically compensating for loss in their efficiency over time” by Shen et al., describes a method and associated system that compensates for long-term variations in the light-emitting efficiency of individual organic light emitting diodes in an OLED display device by calculating and predicting the decay in light output efficiency of each pixel based on the accumulated drive current applied to the pixel and derives a correction coefficient that is applied to the next drive current for each pixel. This patent describes the use of a camera to acquire images of a plurality of equal-sized sub-areas. Such a process is time-consuming and requires mechanical fixtures to acquire the plurality of sub-area images.
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. A grayscale characteristic generating unit generates conversion data having grayscale characteristics obtained by changing the grayscale characteristics of display data that defines the grayscales of pixels with reference to a conversion table whose description contents include correction factors. However, their method requires a large number of 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.
U.S. Pat. No 6,897,842 B2 by Gu, describes using a pulse width modulation (PWM) mechanism to controllably drive a display (e.g., a plurality of display elements forming an array of display elements). A non-uniform pulse interval clock is generated from a uniform pulse interval clock, and then used to modulate the width, and optionally the amplitude, of a drive signal to controllably drive one or more display elements of an array of display elements. A gamma correction is provided jointly with a compensation for initial nonuniformity. However, this technique is only applicable to passive-matrix displays, not to the higher-performance active-matrix displays which are commonly employed.
There is a need, therefore, for a more complete approach for compensating differences between components in electroluminescent displays, and specifically for compensating for initial nonuniformity of such displays.
It is therefore an object of the present invention to compensate for differences in characteristics of a plurality of electroluminescent (EL) subpixels. This object is achieved by a method of compensating for differences in characteristics of a plurality of electroluminescent (EL) subpixels, comprising:
An advantage of this invention is an electroluminescent (EL) display that compensates for differences in characteristics of the EL subpixels composing an EL display, and particularly for the initial nonuniformity of the display, 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, and without confounding the two. This advantageously provides increased signal-to-noise ratio in the compensation measurements. 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 the characteristics of the driving transistor and EL emitter in a subpixel are unique to the specific subpixel and are not impacted by other subpixels that may be open-circuited or short-circuited.
Turning now to
Turning now to
The first electrode of readout transistor 80 is connected to the second electrode of drive transistor 70 and to EL emitter 50. The readout line 30 is connected to the second electrodes of the readout transistors 80 in a column of subpixels 60. Readout line 30 is connected to switch block 130. One switch block 130 is provided for each column of EL subpixels 60. Switch block 130 includes a third switch S3 and a fourth switch S4, and a No-Connect state NC. While the third and fourth switches can be individual entities, they are never closed simultaneously in this method, and thus switch block 130 provides a convenient embodiment of the two switches. The third switch permits current source 160 to be selectively connected to the second electrode of readout transistor 80. Current source 160, when connected by the third switch, permits a predetermined constant current to flow into EL subpixel 60. The fourth switch permits current sink 165 to be selectively connected to the second electrode of readout transistor 80. Current sink 165, when connected by the fourth switch, permits a predetermined constant current to flow from EL subpixel 60 when a predetermined data value is applied to data line 35. Switch block 130, current source 160, and current sink 165 can be located on or off the EL display substrate.
In an EL display including a plurality of EL subpixels, the single current source and sink are selectively connected through the third and fourth switches, respectively, to the second electrode of each readout transistor in the plurality of EL subpixels. More than one current source or sink can be used provided the second electrode of the readout transistor is selectively connected to either one current source or one current sink, or nothing, at any given time.
The second electrode of readout transistor 80 is also connected to a voltage measurement circuit 170, which measures voltages to provide signals representative of characteristics of EL subpixel 60. Voltage measurement circuit 170 includes an analog-to-digital converter 185 for converting voltage measurements into digital signals, and a processor 190. The signal from analog-to-digital converter 185 is sent to processor 190. Voltage measurement circuit 170 can also include a memory 195 for storing voltage measurements, and a low-pass filter 180 if necessary. Voltage measurement circuit 170 can be connected through multiplexer output line 45 and multiplexer 40 to a plurality of readout lines 30 and readout transistors 80 for sequentially reading out the voltages from 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 60 can be driven simultaneously. The plurality of multiplexers 40 will permit parallel reading out of the voltages from the various multiplexers 40, while each multiplexer 40 would permit sequential reading out of the readout lines 30 attached to it. This will be referred to herein as a parallel/sequential process.
Processor 190 can also be connected to data line 35 by way of a control line 95 and a digital-to-analog converter 155. Thus, processor 190 can provide predetermined data values to data line 35 during the measurement process to be described herein. Processor 190 can also accept display data via data in 85 and provide compensation for changes as will be described herein, thus providing compensated data to data line 35 during the display process.
The embodiment shown in
Transistors such as drive transistor 70 of EL subpixel 60 have characteristics including threshold voltage Vth and mobility μ. The voltage on the gate electrode of drive transistor 70 must be greater than the threshold voltage to enable significant current flow between the first and second electrodes. The mobility relates to the amount of current flow when the transistor is conducting. When using a display with a transistor backplane of low-temperature polysilicon (LTPS) transistors, not all transistors in the display necessarily have identical Vth or mobility values. Differences between characteristics of the various drive transistors in the EL subpixels 60 can result in visible nonuniformity in light output across the surface of a display when all drive transistors are driven by the same gate-source voltage Vgs. Such nonuniformity can include differences in brightness and color balance in different parts of the display. It is desirable to compensate for such differences in the threshold voltage and mobility to prevent such problems. Also, there can be differences in the characteristics of the EL emitters 50, such as efficiency or resistance, which can also cause visible nonuniformity.
The present invention can compensate for differences in characteristics and the resulting nonuniformities at any desired time. However, nonuniformities are particularly objectionable to end users seeing a display for the first time. 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 now to
The relationship between the EL current (which is also the drain-source current through the drive transistor), EL voltage, and threshold voltage at saturation 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, Vgs is voltage difference between gate and source of the drive transistor. For simplicity, we neglect dependence of μ on Vgs. Thus, to produce the same current from subpixels having characteristics 230 and 240, one must compensate for differences in Vth and VEL. It is therefore desirable to measure both changes.
Turning now to
The voltages of the components in each subpixel can be related by:
V1=Vdata−Vgs(Itestsk)−Vread (Eq. 2)
where Vgs(Itestsk) is the gate-to-source voltage that must be applied to drive transistor 70 such that it's drain-to-source current, Ids, is equal to Itestsk. The values of these voltages will cause the voltage at the second electrode of readout transistor 80 (Vout, which is read to provide V1) to adjust to fulfill Eq. 2. Under the conditions described above, Vdata is a set value and Vread can be assumed to be constant. Vgs will be controlled by the value of the current set by current sink 165 and the current-voltage characteristics of drive transistor 70, and will be different for different values of the threshold voltage of the drive transistor. To compensate for mobility variations, two values of V1 must be taken at different values of Itestsk.
The value of the first signal V1 can be recorded for each subpixel with selected values for current sink 165. Then, the subpixel with the maximum V1 (thus the minimum Vgs(testsk), so the minimum Vth) is selected as the first target signal, V1target, from the population of subpixels measured. Alternatively, the minimum or mean of all V1 values, or the results of other functions obvious to those skilled in the art, can be selected as V1target. The measured first signal V1 for each subpixel can then be compared to the first target signal V1target to form a delta ΔV1 for each subpixel, as follows:
ΔV1=−ΔVth=V1−V1target (Eq. 3)
ΔV1 represents the difference in threshold voltage between each subpixel and the target.
Note that the present invention only applies to a plurality of EL subpixels, as a single EL subpixel has no difference in characteristics when there is nothing to compare it to. That is, for a single EL subpixel, V1=V1target, so ΔV1=0 always.
Referring back to
The voltages of the components in each subpixel can be related by:
V2=CV+VEL+Vread (Eq. 4)
The values of these voltages will cause the voltage at the second electrode of readout transistor 80 (Vout, which is read to provide V2) to adjust to fulfill Eq. 4. Under the conditions described above, CV is a set value and Vread can be assumed to be constant. VEL will be controlled by the value of current set by current source 160 and the current-voltage characteristics of EL emitter 50. VEL can be different for different EL emitters 50.
The value of the second signal V2 can be recorded for each subpixel with selected values for current source 160. Then, the subpixel with the minimum VEL (that is, the minimum measured V2) is selected as the second target signal, V2target, from the population of subpixels measured. Alternatively, the maximum or mean, or the results of other functions obvious to those skilled in the art, of all V2 values can be selected as V2target. The measured second signal V2 for each subpixel can then be compared to the second target signal V2target to form a delta ΔV2, as follows:
ΔV2=ΔVEL=V2−V2target (Eq. 5)
ΔV2 represents the difference in EL emitter voltage between each subpixel and the target.
When measuring each EL subpixel in a plurality of EL subpixels, the first signal can be read for all EL subpixels, and then the second signal can be read for all EL subpixels, as shown in
The deltas ΔV1 and ΔV2 in the first and second signals, respectively, of each EL subpixel can then be used to compensate for differences (Step 370) in the characteristics of different EL subpixels 60 in a plurality of EL subpixels, such as EL display. For compensating for differences in current between multiple subpixels, it is necessary to make a correction for ΔVth (related to ΔV1) and ΔVEL (related to ΔV2).
To compensate for the differences in characteristics of EL subpixels 60, one can use the deltas in the first and second signals in an equation of the form:
ΔVdata=f1(ΔV1)+f2(ΔV2) (Eq. 7)
where ΔVdata is an offset voltage on the gate electrode of drive transistor 70 necessary to maintain the desired luminance specified by a selected Vdata, f1(ΔV1) is a correction for differences in threshold voltage, and f2(ΔV2) is a correction for differences in EL resistance. ΔV1 is as given in Eq. 3; ΔV2 is as given in Eq. 5. For example, the EL display can include a controller, which can include a lookup table or algorithm to compute an offset voltage for each EL emitter. For example, f1 can be a linear function since Ids of a drive transistor is determined by Vgs−Vth, so a given Vth change ΔV1 can be compensated for by changing Vdata (which approximately equals Vg) by the same amount. In embodiments having the EL emitter connected to the source terminal of the drive transistor, f2 can also be a linear function for an analogous reason: changing the source voltage changes Vgs by the same amount. For more complex cases, the system can be modeled by techniques known in the art, such as SPICE simulation, and f1 and f2 implemented as lookup tables of precomputed values. To compensate for mobility variations, the two measured V1 values at different Itestsk values can be used to determine an offset and a gain which will map the I-V curve for each subpixel onto a reference I-V curve, selected as the mean, minimum, or maximum of the I-V curves of all subpixels. The offset and the gain can be used to transform Vdata on the reference curve to the equivalent voltage on the transformed curve. This linear transform can account for Vth and mobility differences simultaneously.
The offset voltage ΔVdata is computed to provide corrections for differences in current due to differences in the threshold voltages and mobilities of drive transistors 70 and in the resistances of EL emitters 50. This provides a complete compensation solution. These changes can be applied by the controller to correct the light output to the nominal luminance value desired. By controlling the signal applied to the EL emitter, an EL emitter with a constant luminance output and increased lifetime at a given luminance is achieved. Because this method provides a correction for each EL emitter in a display, it will compensate for differences in the characteristics of the plurality of EL subpixels, and can thus compensate for initial nonuniformity of an EL display having a plurality of EL subpixels.
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 |
|---|---|---|---|
| 6081073 | Salam | Jun 2000 | A |
| 6414661 | Shen et al. | Jul 2002 | B1 |
| 6456016 | Sundahl et al. | Sep 2002 | B1 |
| 6473065 | Fan | Oct 2002 | B1 |
| 6897842 | Gu | May 2005 | B2 |
| 7859501 | Levey et al. | Dec 2010 | B2 |
| 7956830 | Chen et al. | Jun 2011 | B2 |
| 20030122813 | Ishizuki et al. | Jul 2003 | A1 |
| 20050007392 | Kasai et al. | Jan 2005 | A1 |
| 20060158402 | Nathan et al. | Jul 2006 | A1 |
| 20060273997 | Nathan et al. | Dec 2006 | A1 |
| 20070195020 | Nathan et al. | Aug 2007 | A1 |
| 20080048951 | Naugler, Jr. et al. | Feb 2008 | A1 |
| 20080252568 | Kwon | Oct 2008 | A1 |
| Number | Date | Country |
|---|---|---|
| WO 2005109389 | Nov 2005 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 20100103082 A1 | Apr 2010 | US |
