An active-matrix organic light-emitting-diode pixel circuit for the compensation of I x R voltage drop in power line

A new voltage-driving active-matrix organic light-emitting diode ( AMOLED) pixel circuit is proposed to improve the display image-quality of AMOLED displays. Because OLEDs are current-driven devices, the I x R voltage drop in the power lines is evitable. Accordingly, the I x R voltage-drop compensation scheme should be included in the pixel-driving method when a voltage-compensation method is used. The proposed pixel was designed for the compensation of an I x R voltage drop in the power lines as well as for the compensation of the threshold-voltage non-uniformity of low-temperature polycrystalline-silicon thin-film transistors ( LTPS TFTs). In order to verify the compensation ability of the proposed pixel, SPICE simulation was performed and compared with those of other conventional pixels. When the V-ss voltage varies from 0 to 1 V, the drain current of the proposed pixel decreased by under 1% while that of conventional V-th compensation methods without V-ss compensation decreased by over 60%. 2.2-in. QCIF(+) full-color AMOLED displays, which employ the proposed pixel, have been also developed. It was verified by comparison of the display image quality with a conventional panel that our proposed panel successfully overcame the voltage-drop problems in the power lines.