LTPO-TFT-Based Pixel Circuit With TFT, OLED, and Supply Voltage Compensation for Enhanced Luminance Uniformity in Variable-Frame-Rate AMOLED Smartwatch Displays

This work proposes a new pixel circuit using low-temperature polycrystalline silicon and oxide (LTPO) thin-film transistors (TFTs) for use in variable-frame-rate active-matrix organic light-emitting diode (AMOLED) smartwatch displays. The proposed circuit, including seven TFTs and two capacitors, can compensate for threshold voltage (VTH) variations of the driving TFTs (DTFTs), the turn-on voltages of the OLEDs (VOLEDs), and VSS IR rises; it is immune to VDD IR drops. It employs amorphous-indium-gallium-zinc-oxide (a-IGZO) TFTs, which exhibit low leakage currents, suppressing distortion in the gate voltage of the DTFT at low frame rates and enabling a stable OLED current (IOLED) to maintain consistent display luminance. A 1.28-inch LTPO AMOLED panel with a 416 W 416 resolution and the proposed pixel circuit are fabricated to verify the circuits performance. The experimental results thus obtained confirm that red, green, blue, and white images at frame rates from 45 Hz to 1 Hz exhibit uniformity without visible spot or line defects and a luminance error rate below 2.34 remain stable at gray levels of 32, 64, 128, and 255 during an extended emission period of 1 s, revealing no perceived image flicker at 1 Hz (with a JEITA flicker value below -54.747 dB). Therefore, the proposed circuit, with its highly uniform and stable currents at various frame rates, is promising for use in AMOLED smartwatch displays.