Recently active-matrix organic light-emitting diode (AMOLED) displays with amorphous silicon (a-Si), poly-silicon, organic, or other driving backplane have become more attractive due to advantages over active matrix liquid crystal displays. For example, the advantages include: with a-Si besides its low temperature fabrication that broadens the use of different substrates and makes feasible flexible displays, its low cost fabrication, high resolution, and a wide viewing angle.
An AMOLED display includes an array of rows and columns of pixels, each having an organic light-emitting diode (OLED) and backplane electronics arranged in the array of rows and columns. Since the OLED is a current driven device, the pixel circuit of the AMOLED should be capable of providing an accurate and constant drive current.
U.S. Pat. No. 6,594,606 discloses a method and system for calibrating passive pixels. U.S. Pat. No. 6,594,606 measures data line voltage and uses the measurement for pre-charge. However, this technique does not provide the accuracy needed for active matrix, since the active matrix calibration should work for both backplane aging and OLED aging. Further, after pre-charge, current programming must be performed. Current-programming of current driven pixels is slow due to parasitic line capacitances and suffers from non-uniformity for large displays. The speed may be an issue when programming with small currents.
Other compensation techniques have been introduced. However, there is still a need to provide a method and system which is capable of providing constant brightness, achieving high accuracy and reducing the effect of the aging of the pixel circuit.
Amorphous silicon is, for example, a promising material for AMOLED displays, due to its low cost and vast installed infrastructure from thin film transistor liquid crystal display (TFTLCD) fabrication.
All AMOLED displays, regardless of backplane technology used, exhibit differences in luminance on a pixel to pixel basis, primarily as a result of process or construction inequalities, or from aging caused by operational use over time. Luminance non-uniformities in a display may also arise from natural differences in chemistry and performance from the OLED materials themselves. These non-uniformities must be managed by the AMOLED display electronics in order for the display device to attain commercially acceptable levels of performance for mass-market use.
FIG. 1B illustrates an operational flow of a conventional AMOLED display 10. Referring to FIG. 1B, a video source 12 contains luminance data for each pixel and sends the luminance data in the form of digital data 14 to a digital data processor 16. The digital data processor 16 may perform some data manipulation functions, such as scaling the resolution or changing the color of the display. The digital data processor 16 sends digital data 18 to a data driver integrated circuit (IC) 20. The data driver IC 20 converts that digital data 18 into an analog voltage or current 22, which is sent to thin film transistors (TFTs) 26 in a pixel circuit 24. The TFTs 26 convert that voltage or current 22 into another current 28 which flows through an organic light-emitting diode (OLED) 30. The OLED 30 converts the current 28 into visible light 36. The OLED 30 has an OLED voltage 32, which is the voltage drop across the OLED. The OLED 30 also has an efficiency 34, which is a ratio of the amount of light emitted to the current through the OLED.
The digital data 14, analog voltage/current 22, current 28, and visible light 36 all contain the exact same information (i.e. luminance data). They are simply different formats of the initial luminance data that came from the video source 12. The desired operation of the system is for a given value of luminance data from the video source 12 to always result in the same value of the visible light 36.
However, there are several degradation factors which may cause errors on the visible light 36. With continued usage, the TFTs will output lower current 28 for the same input from the data driver IC 20. With continued usage, the OLED 30 will consume greater voltage 32 for the same input current. Because the TFT 26 is not a perfect current source, this will actually reduce the input current 28 slightly. With continued usage, the OLED 30 will lose efficiency 34, and emit less visible light for the same current.
Due to these degradation factors, the visible light output 36 will be less over time, even with the same luminance data being sent from the video source 12. Depending on the usage of the display, different pixels may have different amounts of degradation.
Therefore, there will be an ever-increasing error between the required brightness of some pixels as specified by the luminance data in the video source 12, and the actual brightness of the pixels. The result is that the decreased image will not show properly on the display.
One way to compensate for these problems is to use a feedback loop. FIG. 2B illustrates an operational flow of a conventional AMOLED display 40 that includes the feedback loop. Referring to FIG. 2B, a light detector 42 is employed to directly measure the visible light 36. The visible light 36 is converted into a measured signal 44 by the light detector 42. A signal converter 46 converts the measured visible light signal 44 into a feedback signal 48. The signal converter 46 may be an analog-to-digital converter, a digital-to-analog converter, a microcontroller, a transistor, or another circuit or device. The feedback signal 48 is used to modify the luminance data at some point along its path, such as an existing component (e.g. 12, 16, 20, 26, 30), a signal line between components (e.g. 14, 18, 22, 28, 36), or combinations thereof.
Some modifications to existing components, and/or additional circuits may be required to allow the luminance data to be modified based on the feedback signal 48 from the signal converter 46. If the visible light 36 is lower than the desired luminance from video source 12, the luminance signal may be increased to compensate for the degradation of the TFT 26 or the OLED 30. This results in that the visible light 36 will be constant regardless of the degradation. This compensation scheme is often known as Optical Feedback (OFB). However, in the system of FIG. 2B, the light detector 42 must be integrated onto a display, usually within each pixel and coupled to the pixel circuitry. Not considering the inevitable issues of yield when integrating a light detector into each pixel, it is desirable to have a light detector which does not degrade itself, however such light detectors are costly to implement, and not compatible with currently installed TFT-LCD fabrication infrastructure.
Therefore, there is a need to provide a method and system which can compensate for non-uniformities in displays without measuring a light signal.
AMOLED displays are conventionally operated according to digital data from a video source. The OLEDs within the display can be programmed to emit light with luminance according to a programming voltage or a programming current. The programming current or programming voltage are conventionally set by a display driver that takes digital data as input and has an analog output for sending the programming current or programming voltage to pixel circuits. The pixel circuits are configured to drive current through OLEDs based on the programming current or programming voltage.