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

Abstract

AbstractA 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.