An AMOLED Pixel Circuit With a Compensating Scheme for V ariations in Subthreshold Slope and Threshold V oltage of Driving TFTs

Abstract

This article proposes an active-matrix organic light- emitting diode (AMOLED) pixel circuit for mobile displays with high resolution to improve the image quality of the display panel. The proposed pixel circuit, which consists of six low-temperature polycrystalline silicon thin-film transistors (TFTs) and two capac- itors, compensates for variations in the threshold voltage (Vth) and subthreshold slope (SS) of driving TFTs at low gray levels. Such compensations are achieved by extending the compensation time in the diode-connection scheme, resulting in a reduction in the OLED current error (OCE) of the proposed pixel circuit. In addition, the proposed pixel circuit connects two capacitors in parallel to the gate node of the driving TFT, thus increasing the total storage capacitance value in a given capacitor area in the emission phase, resulting in a reduction in voltage distortion at the gate node. Moreover, the OCEs caused by the IR drop of ELVDD and the crosstalk due to parasitic capacitances are thoroughly investigated and analyzed. The measurement results of the proposed pixel circuit show that the OCEs at the 64th and 128th gray levels are reduced from ±3.5 and ±6.5 to ±2.0 and ±0.8 LSB, respectively, by extending the compensation time from 6.5 to 195 µs. Therefore, the proposed pixel circuit is highly suitable for mobile AMOLED displays requiring high image quality.