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

Abstract

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 ( $V_{\mathrm { TH}}$ ) variations of the driving TFTs (DTFTs), the turn-on voltages of the OLEDs ( $V_{\mathrm { OLED}}$ ), and $V_{\mathrm { SS}}$ IR rises; it is immune to $V_{\mathrm { DD}}$ 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 ( $I_{\mathrm { OLED}}$ ) to maintain consistent display luminance. A 1.28-In LTPO AMOLED panel with a $416\times 416$ resolution and the proposed pixel circuit are fabricated to verify the circuit’s performance. The experimental results thus obtained confirm that red, green, blue, and white images at frame rates from 45 to 1 Hz exhibit uniformity without visible spot or line defects and a luminance error rate below 2.34%. Measured luminance values 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 Japan Electronics and Information Technology Industries Association 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.