Correlation between the compensation time and the current deviation of organic LED pixel circuit

Abstract

A compensation circuit is necessary in each pixel of the organic light‐emitting diode (OLED) display to reduce the mura resulting from the nonuniform characteristics of the low‐temperature polycrystalline silicon thin‐film transistor (TFT). Recently, the mura becomes severe as the frame rate, display resolution, and the OLED efficacy improve. The low‐gray mura for the OLED current far below 1 nA is easily noticed, which is a critical issue of the high‐end mobile OLED display. We report the correlation between the deviation of OLED current due to the nonuniform TFT characteristics and the compensation time by in‐depth analysis employing the SPICE simulation of two well‐known pixel circuits. We have found that the compensation effect improves as the compensation time increases for low gray‐level current. On the contrary, the compensation effect deteriorates, and the deviation of OLED current intensifies for high gray‐level current as the compensation time increases too much. The quantitative analysis shows that the optimal compensation time depends on the targeted OLED current level and that the compensation effect is highest when the compensation time is such that the current of the driving TFT at the end of compensation period is several times smaller than the targeted OLED current during the light‐emitting period.

P‐10: The Novel OLED Compensation TFT Circuit for Enhanced Outside Visibility

“…1 In a conventional compensation pixel circuits, the maximum current of the driving TFT increases by increasing the Vdata range or increasing the width/length (W/L) ratio of the driving TFT. [2][3][4] However, the power consumption of the driving integrated circuit (IC) sharply increases by increasing Vdata range. 5 This causes serious power loss of the display module.…”

“…2. [2][3][4] In the bright mode, the voltage of VINIT2 is same with VINIT. In this mode, 11T2C circuit can be considered as parallel connected two conventional circuits sharing T2 and T3.…”

“…In the dark mode, VINIT2 is equal to VDD, so DTFTm is always off. Therefore, the circuit operates just as a conventional diode-connection pixel compensation circuit which is the partial circuit shown by the red dashed line in fig.2 [2][3][4]. In the bright mode, the voltage of VINIT2 is same with VINIT.…”

Analog PWM Method With Sweep Generation Structure Based on P-Type LTPS TFTs for Micro-LED Displays

“…However, to drive the micro-LED in a high current over the long term, LTPS TFTs, which possess excellent mobility and reliability, are preferred for micro-LED display [3], [4]. Although the threshold voltage (VTH) variations of LTPS TFTs influence the emission currents and generate mura defects in the displays [5], many pulse amplitude modulation (PAM) circuits for pixels have been proposed to address the VTH variation issue [6]- [8]. Lee et al presented a 7T1C PAM circuit that can sense and compensate for the VTH of driving TFT (T1) in Fig.…”

“…Lee et al presented a 7T1C PAM circuit that can sense and compensate for the VTH of driving TFT (T1) in Fig. 1 [8]. Nevertheless, the 7T1C circuit is unsuitable for micro-LED displays because the emission wavelength of micro-LED varies with current density [9].…”