Patent Application
20100277400
1. Field of the Invention
The present invention relates to calibration of current variations in the pixels/sub-pixels of an active matrix organic light-emitting diode (AMOLED) display caused by aging of the organic light-emitting diodes (OLEDs) in the AMOLED sub-pixels.
2. Description of the Related Arts
An OLED display is generally comprised of an array of organic light emitting diodes (hereafter referred to as “OLED diodes”) that have carbon-based films deposited between two charged electrodes. Generally one electrode is comprised of a transparent conductor, for example, indium tin oxide (ITO). Generally, the organic material films are comprised of a hole-injection layer, a hole-transport layer, an emissive layer and an electron-transport layer. When voltage is applied to the OLED diode, the injected positive and negative charges recombine in the emissive layer and transduce electrical energy to light energy. Unlike liquid crystal displays (LCDs) that require backlighting, OLED displays are self-emissive devices—they emit light rather than modulate transmitted or reflected light. Accordingly, OLEDs are brighter, thinner, faster and lighter than LCDs, and use less power, offer higher contrast and are cheaper to manufacture.
An OLED display typically includes a plurality of OLED diodes arranged in a matrix form including a plurality of rows and a plurality of columns, with the intersection of each row and each column forming a pixel of the OLED display. An OLED display is generally activated by way of a current driving method that relies on either a passive-matrix (PM) scheme or an active-matrix (AM) scheme.
In a passive matrix OLED (PM OLED) display, a matrix of electrically-conducting rows and columns forms a two-dimensional array of picture elements called pixels. Sandwiched between the orthogonal column and row lines are thin films of organic material of the OLEDs that are activated to emit light when current is applied to the designated row and column lines. The brightness of each pixel is proportional to the amount of current applied to the OLED diodes of the pixel. While PM OLEDs are fairly simple structures to design and fabricate, they demand relatively expensive, current-sourced drive electronics to operate effectively and are limited as to the number of lines because only one line can be on at a time and therefore the PM OLED must have instantaneous brightness equal to the desired average brightness times the number of lines. Thus, PM OLED displays are typically limited to under 100 lines. In addition, their power consumption is significantly higher than that required by an active-matrix OLED. PM OLED displays are most practical in alpha-numeric displays rather than higher resolution graphic displays.
An active-matrix OLED (AMOLED) display is comprised of OLED pixels (that are each comprised of R, G, B sub-pixels) that have been deposited or integrated onto a thin film transistor (TFT) array to form a matrix of pixels that emit light upon electrical activation. In contrast to a PM OLED display, for which electricity is distributed row by row, the active-matrix TFT backplane acts as an array of switches coupled with sample and hold circuitry that control and hold the amount of current flowing through each individual OLED sub-pixel during the total frame time. The active matrix TFT array continuously controls the current that flows to the OLED diodes in each of the sub-pixels, signaling to each pixel how brightly to illuminate.
AMOLED displays require regulated current in each pixel to produce a desired brightness from the pixel. Ideally, the TFTs in the active matrix TFT array exhibit uniform electrical characteristics, so that the AMOLED display can be precisely controlled in a uniform manner. However, the TFTs in the AMOLED are typically fabricated with poly-silicon (p-Si) that is difficult to fabricate in a uniform manner. This is because p-Si is made by converting amorphous silicon (a-Si) to p-Si by laser annealing the a-Si to increase the crystal grain size. The larger the crystal grain size, the faster and more stable is the resulting semiconductor material. Unfortunately the grain size produced in the laser anneal step is not uniform due to a temperature spread in the laser beam. Thus, uniform TFTs are very difficult to produce and thus the current supplied by TFTs in conventional AMOLED displays is often non-uniform, resulting in non-uniform display brightness. TFT non-uniformity throughout the OLED display causes “Mura” (streaking or spots) in the OLED displays made with p-Si TFTs. In other words, TFTs may produce different OLED currents due to their non-uniformities from pixel to pixel, even if the same gate voltage is applied to the TFTs.
Another problem with AMOLED displays occurs due to aging of the material in the OLEDs. As the OLED diode in each sub-pixel ages with use, it becomes less efficient in converting current to light, i.e., the efficiency of light emission of the OLED diode decreases. Thus, as OLED diode current to light efficiency of the OLED material decreases with use (age), light (luminance) emitted from an OLED diode in each sub-pixel for a given gate voltage applied to the drive TFTs of the OLED display also decreases. As a result, the OLED display emits less light for display than desired in response to a given gate voltage applied to the drive TFTs. In addition, since the OLED diodes on various parts of the AMOLED display do not age (are not used) equally in a uniform manner, OLED aging also causes non-uniformity in the OLED display. In addition, since aging is accelerated at higher currents, a repeating image at a high gray level will appear to remain or stick on the AMOLED panel, hence the term “image sticking” due to aging. As a result of aging, the forward voltage VF of an OLED in a sub-pixel required to generate a given OLED current will increase. Also, given an OLED current, the luminance from the OLED will decrease. The present invention seeks to correct such problems in the AMOLED display that arise from aging of the OLEDs in the AMOLED sub-pixels.
According to various embodiments of the present invention, the data line voltage on the data line of the AMOLED sub-pixels is measured while the OLED is being driven by a reference current in order to determine the age of the OLED in the sub-pixel. The pixel transistor serves as a current source for driving the OLED in the sub-pixel with the reference current. The data line voltage is substantially equal to the forward voltage VF(aged) of the aged OLED being driven at the reference current. The forward voltage VF (un-aged) of a reference (un-aged) OLED sub-pixel also measured at the reference current, and is subtracted from the measured OLED diode forward voltage VF (aged) to obtain their difference ΔVF=VF(aged)−VF(un-aged). ΔVF is an indicator of the age of the OLED in the sub-pixel, and is used as an index to a look-up-table that stores the corresponding aging offset data for generating the incremental pixel current needed to maintain constant luminance in the aged OLED pixel.
The features and advantages described in the specification are not all inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings and specification. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and may not have been selected to delineate or circumscribe the inventive subject matter.
The teachings of the embodiments of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings.
The Figures and the following description relate to preferred embodiments of the present invention by way of illustration only. It should be noted that from the following discussion, alternative embodiments of the structures and methods disclosed herein will be readily recognized as viable alternatives that may be employed without departing from the principles of the claimed invention.
Reference will now be made in detail to several embodiments of the present invention(s), examples of which are illustrated in the accompanying figures. It is noted that wherever practicable similar or like reference numbers may be used in the figures and may indicate similar or like functionality. The figures depict embodiments of the present invention for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the invention described herein.
The source of TFT M2 is connected to data line D, and the drain of TFT M2 is connected to the gate of TFT M1 (the “pixel transistor”) and to one side of storage capacitor C0. The source of TFT M1 is connected to positive supply voltage ELVDD. The other side of storage capacitor C0 is also connected, for example, to the positive supply voltage ELVDD and to the source of TFT M1. Note that the storage capacitor C0 may be tied to any reference electrode in the pixel, but the connection shown in
When TFT M2 is turned on, the analog gate voltage from the data line D is applied to the gate of each TFT M1 of each sub-pixel, which is locked by storage capacitor C0. In other words, the continuous current flow to the OLED diodes is controlled by the two TFTs M1, M2 of each sub-pixel. TFT M2 is used to start and stop the charging of storage capacitor C0, which provides a voltage source to the gate of TFT M1 at the level needed to create a constant current to the OLED diode. The TFT M2 samples the data on the data line D, which is then transferred to and held by the storage capacitor C0. The voltage held on the storage capacitor C0 is applied to the gate of the TFT M1. In response, TFT M1 drives current through the OLED diode D0 to a specific brightness depending on the value of the sampled and held voltage as stored in the storage capacitor C0.
In addition to the two TFTs M1, M2 typically found in conventional AMOLED cells (“2T cell structure”), the AMOLED sub-pixel of the present invention employs a “3T cell structure” that additionally includes a third TFT M3 with one additional control line S that can be used to control the gate voltage of TFT M3. As will be explained in more detail below, TFT M3, when turned on, enables the forward voltage of OLED D0 to be measured via the data line D. Thus, the AMOLED display of the present invention uses “data line sensing” to sense the OLED forward voltage. As shown in
Turning to the OLED aging problem, as mentioned briefly above, OLEDs age over time, resulting in increase of the forward voltage VF across OLED diode D0 for a given OLED diode current (If). Also, even if the OLED diode is operated at constant current (If), the luminance from the OLED diode will decrease as a result of aging. Since aging is accelerated at higher currents, a repeating image at a high gray level will appear to remain or stick on the AMOLED panel, hence the term “image sticking” due to aging. By measuring the forward voltage (VF) across the OLED diode D0 at a constant current and temperature for each pixel over time as the OLED diode ages, the amount of lost luminance from OLED aging can be inferred from ΔVF, i.e., the change in the OLED diode forward voltage (VF) over time at a constant OLED diode current (If) as the AMOLED display ages. Alternatively, the OLED diode forward voltage VF(un-aged) of an un-aged OLED diode can be measured and then this value can be subtracted from the measured OLED diode forward voltage VF(aged) of an aged, active sub-pixel to obtain ΔVF, i.e., ΔVF=VF(aged)−VF(un-aged), which method is preferred since it cancels temperature dependence. Then, as will be explained in more detail below with reference to
As can be seen above, the success of the aging compensation technique depends upon the ability to measure the OLED diode forward voltage VF at a constant current over time as the AMOLED ages. It is expected that 5 or more “image sticking” calibrations should be performed over the lifetime of the AMOLED product. Such calibrations may occur at, for example, 100 hours, 200 hours, 300 hours, 500 hours, and 1000 hours of use (depending upon the lifetime of the display).
Current setting logic 412 includes logic that is configured to drive column DACs 306 with a reference voltage VREF that corresponds to reference current IREF, so that the OLED D0 in the sub-pixel in the AMOLED panel to be corrected for aging is driven with the reference current IREF. The reference current IREF is the constant current to be used for measuring the OLED forward voltage VF. The value of reference current IREF may differ depending on the size of the AMOLED panel. In one embodiment, the reference current IREF is 200 nA. In another embodiment, reference current IREF is 1 μA. The reference voltage VREF is provided to the sub-pixels through MUX 302 by turning on the switches SW1 (and turning off switches SW2) in MUX 302 via the data lines D of the sub-pixel to be calibrated. On the other hand, when the switches SW2 are turned on (and switches SW1 are turned off), the voltage (Vdata) 402 on the data line of the sub-pixel to be calibrated becomes coupled to aging calibration circuit 400 for measurement. As will be explained in more detail below with reference to
The operation of
In step 604, the scan transistors M2 of the sub-pixels to be calibrated in the selected row are turned off. Then, in step 606, the voltage on the data lines D of the sub-pixels to be calibrated in the selected row are driven to the OLED forward voltage VF of an un-aged pixel corresponding to the reference current IREF, using current setting logic 412. This OLED forward voltage VF of an un-aged pixel may have been measured previously using the same techniques as described in
In step 608, the sense transistor M3 of the sub-pixels to be calibrated on the selected row are turned on, and the process waits until the data line D of the sub-pixels settle to the forward voltage VF of the OLED D0 of the aged sub-pixel. Because the data line D of the sub-pixels is a capacitive load, once the data line D settles to the forward voltage VF of the OLED D0, all the current from the pixel transistor M1 flows through the OLED D0 and no current flows through the sense transistor M3, RGB MUX (not shown), and data line D. Thus, the voltage on the data line D becomes substantially equal to the forward voltage VF of the OLED D0, since there is no voltage drop on the data line D.
Then, in step 610, the voltage on the data line D of the sub-pixels is measured using ADC 404, as explained above. During step 610, switches SW1 are opened and switches SW2 are closed in the MUX 302 to disconnect the column DACs 306 from the data line D and connect the data line D to the aging calibration circuit 400. While steps 602, 604, 606, and 608 may be performed on all or multiple sub-pixels of the selected row of the AMOLED panel, step 610 is performed on each sub-pixel one at a time if there is only a single calibration circuit 400 with the ADC 404. Alternatively, the calibration circuitry 400 can be configured to include multiple ADCs 404 to measure the voltage on data line D of multiple sub-pixels at a time, in order to enhance the speed of image sticking calibration. As explained above, the measured data line voltage in step 610 is the forward voltage VF (aged) 714 of the aged sub-pixel, which is then compared with the forward voltage VF (un-aged) 716 of the un-aged sub-pixel to determine the difference ΔVF 712 between VF (aged) and VF (un-aged). ΔVF 712 is stored in look-up table 408 and converted to ΔGrayScale values indicating the aging offset data for storage in aging offset RAM 704.
By performing steps 602, 604, 606, 608, and 610, the aging calibration process for one selected row of the AMOLED panel is completed. These steps 602, 604, 606, 608, and 610 can be repeated for other rows of the AMOLED panel, row by row, to complete the aging calibration process for the entire AMOLED panel. At the end of that process, the aging offset RAM 704 would store aging offset data (ΔGrayScale values) for each of the sub-pixels of the entire AMOLED panel.
The circuitry and method for measuring the forward voltage VF of OLEDs as described in
For aging compensation, the un-aged OLED diode forward voltage VF (un-aged) 716 of each sub-pixel for un-aged sub-pixels conducting the predetermined constant OLED diode current (IREF) may be compared with the forward voltage VF(aged) 714 of aged OLEDs needed to have the same predetermined constant OLED current (IREF) flow in aged OLEDs D0, to determine the difference ΔVF 712 in such forward voltages and infer how aged the OLED D0 is. The forward voltage difference ΔVF 712 is used as an index into a look-up table 710 that stores factory-determined full-scale aging offset data needed to compensate for such aging in the OLEDs as a function of the inferred age of the OLED diode indicated by ΔVF 712. Such aging offset data is stored in the aging offset RAM 704 at a location corresponding to the calibrated sub-pixel.
The data stored for each sub-pixel in the offset RAMs 704 and 706 corresponds to the correction needed for full-scale pixel current (e.g., M1 pixel current Ip=200 nA) which corresponds to a full-scale RGB data. For real-time display, the data in the offset RAMs 704 and 706 should be scaled according to the real-time RGB data so that full-scale offsets are scaled accordingly for less than full-scale RGB input data. Mura offset data scaler 718 and aging offset data scaler 720 scale the full-scale Mura offset data and the full-scale aging offset data, respectively, to correspond to the real-time RGB data 724 for the driven sub-pixel. Adder 722 performs real-time addition of the scaled Mura offset value 732 and the scaled aging (image sticking) offset value 734 to the real-time RGB data 724 corresponding to the driven sub-pixel, and the summed result is stored temporarily in column DAC registers 702 as compensated RGB data for driving the column DAC 306 that subsequently drives the sub-pixels for real-time display. Thus, the OLED sub-pixels will illuminate light calibrated for Mura and especially for aging, as determined by the process illustrated above in
Upon reading this disclosure, those of skill in the art will appreciate still additional alternative designs for correction of aging in AMOLED displays. Thus, while particular embodiments and applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited to the precise construction and components disclosed herein and that various modifications, changes and variations which will be apparent to those skilled in the art may be made in the arrangement, operation and details of the method and apparatus of the present invention disclosed herein without departing from the spirit and scope of the present invention.
