Pixel Circuit Design Of OLED

The active-matrix organic light-emitting diode (AMOLED) which can achievehigh-resolution and large-size panel display is a new flat panel display technology after LCDflat panel display, due to their superior advantages such as fast response time, low operatingvoltage, wide viewing angle and flexibility. Unlike the liquid crystal, the OLED requires astable current to control the emission luminance whereas the conventional pixel circuitcomposed of two thin film transistors (TFT) and a capacitor (2T1C) can’t guaranteeuniformity requirements of the displays. Thus the research of the pixel circuits with a functionof threshold voltage compensation is one of the key problems of the AMOLED displaytechnology.Firstly, this paper studys of common problems in AMOLED pixel design.To achieve thewhite balance, RGB sub-pixel emission luminance will follow a certain brightness ratio, thispaper analyzes and calculates the white balance of how to achieve a certain emissionluminance of the display panel. Meanwhile, combinated with MOTFT and OLED deviceparameters, we give the method to select the design parameters based on the circuit of2T1C,which provides a reference for selecting parameters of compensation pixel circuits. And weanalyzed the non-uniformity of the2T1C pixel circuit,then followed by the experimentverification.Theoretical analysis and experimental verification show that, traditionalAMOLED display with2T1C structure is not suitable for high-quality commercial displayapplications. And it is essential to develop pixel circuits with compensation function.To compensate for the TFT threshold voltage shift and the degradation of OLED,wedesigned the voltage programming5T2C pixel circuit. Compared with the previous similarcircuits, the OLED does not emit light during the programming period, which can achieve ahigh contrast ratio. In order to slow down the degradation of the OLED devices, we designedan AC driving4T1C pixel. In each frame, OLED is reverse-biased for some time to restorethe characteristic of the OLED. Compared with the5T2C pixel circuit in this paper, the ACdriving4T1C pixel circuit eliminates a TFT and a capacitor, which can achieve a higheraperture ratio (44%). To compensate for the drift of the TFT mobility, we designed a current programming4T1C pixels circuit. Compared with the previous similar circuit, the currentprogramming4T1C circuit can speed up the charging speed by a clever capacitor connection,thereby reducing the programming time.For each new pixel circuit designed in this paper, we analysed the effect on the circuit bythe speed, power and parasitic parameters, as well as the key factors affecting thenon-uniformity of the emission current, which has a reference value for AMOLED paneldesign and analysis.