Ion-Charge Effects on the Physical Properties of Liquid Crystal Cells

• Main Authors: Po-Lun Chen, 陳伯綸
• Other Authors: Shu-Hsia Chen
• Format: Others
• Language: en_US
• Published: 2000
• Online Access: http://ndltd.ncl.edu.tw/handle/11086773409099298533

博士 === 國立交通大學 === 光電工程所 === 88 === Influence of ion-charge effects on the physical properties of liquid crystal cells is seldom discussed systematically. Many simpler models were proposed for the explanation of specific observed phenomena, while these models are not well equivalent effectively. The dissertation starts with the discussion of ionic charge mechanisms, models, and measurements. And then, I aim to some phenomena involved the ion-charge effects that can not be explained by the conventional models. First, time dependent optical characteristics are studied. I establish a numerical analysis tool for calculating the orientation of liquid crystals. The theory combines the motion equations of ionic charges with the elastic and electric free energies. Furthermore, the extended Jones matrix method is carried out for analyzing the optical behavior of a liquid crystal cell. It gives results for explaining the reversed ripple phenomenon of a twisted nematic liquid crystal cell driven at low frequency. The reversed ripple can not be explained by both of the electric double layer and the equivalent RC-circuit models. In addition, the influence of ion-charge effects on the gray level shift of a liquid crystal cell is also discussed. From my experiment, it is suggested that a reversed gray level shift occurs when a LC cell is doped with a proper concentration of ionic charges. The effect, on contrary to the DC screening effect, promotes liquid crystals to over respond to the applied voltage. Second, time dependent capacitance characteristics are studied. I found that a liquid crystal cell containing large amounts of ionic charges under a DC drive results in anomalous capacitance behaviors. I proposed a general capacitance theory of liquid crystal cells. The capacitance of a liquid crystal cell consists of the terms from the induced polarization and the derivative of space charge with respect to the AC voltage. Finally, I propose an effective method for evaluating the image-sticking effect of a thin film transistor liquid crystal display module by directly measuring its time evolution of transmission. The method can overcome the problem in conventional methods to which the instrumental measurement is difficult to be applied.