<title>Ionic effects in LCDs</title>

Abstract

In this paper we give an overview of the effects of ionic impurities present in LCD's. First we discuss the case of Active Matrix addressed nematic LCD's (AMLCD) as these are at this moment the most interesting candidates for high preformance flat display panels with multi-colour possibilities. As a second type we consider the Surface Stabilized Ferroelectric LCD's (SSFLCD).1 Active Matrix Twisted Nematic LCD's Liquid crystal displays have been used for many years as cheap, low performance small displays in watches, video games etc. Recently however, also high information content displays became feasible using LCD's. The STN effect already allowed for reasonable performance displays and now the Active Matrix LCD's can combine high resolution with multi colour possibilities.Thanks to intensive research efforts made by the LC-producers, extremely pure LC-mixtures are now available. Intrinsic resistivities of more than 1O'31crn are reached [1]. After processing and operating the LCD however, resistivity values can drop over an order of magnitude or more [2] [3]. A possible explanation for this phenomenon is the introduction of impurities during processing or operating. These impurities, but also the LC-material itself, can ionize partly.Two important parameters of an LCD are the switching speed (possibility to obtain video speed) and the number of grey levels (and thus the number of colours) that can be created. We will explain how the presence of ionic impurities deteriorates the performance of an AMLCD in both respects. Holding Ratio of the AMLCDIn each pixel of an AMLCD, a thin film transistor (TFT) is included as a switching element ( fig. 1)[4] [5]. During one line time the driving voltage is switched on the pixel. During the remaining frame time the TFT is in cut-off and the charge should be retained on the pixel capacitor in order to obtain a controllable r.m.s.-voltage over the pixel. However, when the voltage waveform over a pixel in an AMLCD is actually measured, a voltage drop is noticed ( fig. 2).Because of the dielectric anisotropy e of liquid crystals, the slow reorientation of nematic LC molecules causes a change of the pixel capacitance during the frame time. Since in active matrix addressing the charge on the pixel O-8194-1073-X/93/$6.OO SPIE Vol. 1845 Liquid and Solid State Crystals (1992)! 391 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/03/2015 Terms of Use: http://spiedl.org/terms