An a-IGZO TFT Based Pixel Circuit Suitable for Stable Operation Under Amoled Impulsive Driving Dimming Scheme
Authors: Lee Ji-Ho , Park Ji-Hwan , Kang Kyeong-Soo , Park Chanjin , Lee Soo-Yeon 
Published: December 2024

Active matrix organic light emitting diode (AMOLED) display backplanes based on amorphous indium-gallium-zinc oxide (a-IGZO) TFTs have gained considerable attention [1]-[3], due to their large on-off ratio, extremely low off-current, and low-temperature fabrication process [4]-[6].
In mobile AMOLED displays, AMOLED impulsive driving (AID) dimming scheme is required to finely tune the overall brightness of the display [7]. This AID dimming modifies the off-duty ratio of the emission (EM) signal of the pixel circuit according to ambient light conditions, thereby regulating the brightness scale of the screen. Consequently, the phase of the EM signal is altered more than once during one frame [8].
However, the multiple switching of the EM signal within a single frame can lead to undesired voltage loss in capacitors responsible for storing the threshold voltage (VTH) of a driving transistor and the data voltage related to the luminance of OLED. Such voltage loss ultimately results in luminance non-uniformity of the display, especially in low gray levels [9]. Therefore, it is important to design a pixel circuit minimizing the effect of luminance distortion under repetitive switching.
In this work, we proposed a pixel circuit based on a-IGZO TFTs, comprising 6 transistors and 2 capacitors (6T2C). The schematic of the proposed pixel circuit is shown in Figure 1(a). Transistor 1 (T1) serves as the driving transistor, supplying current to the OLED, while T2 to T6 are the switching transistors. Because only the source node of the driving transistor is affected by the EM signal, additional voltage loss in capacitors caused by several switching operations can be diminished. Furthermore, two capacitors are employed to mitigate low compensation accuracy due to the short addressing time under a high refresh rate (e.g., 1.9 μs for 240 Hz refresh rate with a UHD resolution (3080 x 2160) display). Hence, the proposed pixel circuit shows stable operation under the AID dimming scheme.
Figure 1(b) shows the HSPICE simulation result of the OLED current error rate degradation before and after implementing the AID dimming. The proposed circuit is compared with a-IGZO TFT based 7T2C pixel circuit, which the EM signal affects both the gate and source nodes of the driving transistor [3]. The current error rate of the 7T2C pixel circuit increased by over half at lower gray levels when the AID dimming was applied. In contrast, the proposed pixel circuit showed negligible variation in current error rate across all gray levels after adopting the AID dimming scheme.
The simulation result indicates the proposed pixel circuit can maintain the luminance uniformity under the AID dimming without additional compensation. This demonstrates the potential for the development of mobile AMOLED display backplane technologies based on a-IGZO TFTs.

References

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[9] J.-H. Lee, K.-S. Kang, J.-H. Park, C. Park, and S.-Y. Lee, "Multiple-Threshold-Voltage-Compensation Driving Scheme Using the Dimming Period to Stabilize the OLED Luminance at Low Gray Levels." IEEE 2023 30th International Workshop on Active-Matrix Flat Panel Displays and Devices (AM-FPD), pp. 222-225, 2023.

Figure 1