離子電荷效應對液晶盒物理特性之影響

Abstract

離子電荷效應對於液晶盒物理特性的影響, 鮮有文獻做有系統的歸納探討。各種較簡易的模型被提出以解釋各別的觀察現象, 但各模型卻不能有效的對應。本論文先就不同的離子電荷機制、模型、量測方式檢討起, 然後針對現有的文獻所提模型不足以解釋但又和離子電荷效應有關的一些觀察現象做討論。 首先, 針對時變的光學特性討論。建立了一套結合液晶彈性自由能、外加電場自由能和離子電荷運動方程的液晶指向矢動態數值分析工具, 再配合廣義瓊斯矩陣法 (Extended Jones matrix method), 解釋了傳統的電雙層模型和電阻-電容等效電路模型所不能說明的低頻操作下扭轉向列式 (twisted nematic) 液晶盒的光學反漣波 (reversed ripple) 現象。此外, 亦討論離子電荷效應對於液晶盒的光學灰階偏移 (gray level shift) 的影響。由實驗得知液晶盒若含有適當的離子電荷密度, 在較高頻率電壓驅動下, 會使其光學灰階朝過反應於外加電壓的狀態偏移。此現象和直流遮避效應(DC screening effect)的驅勢相反, 亦用數值分析探討之。 其次, 針對時變的電容特性討論。發現了含有較多離子電荷的液晶盒, 在較高的直流電壓作用下, 所產生的異常電容現象。提出了一個通用的液晶盒電容模型, 說明了液晶盒電容除了傳統考慮的液晶分子誘發極化 (induced polarization) 項之外, 還需加入一項由空間電荷 (space charge) 對交流電容偵測電壓微分所造成的電容項。 最後, 針對於薄膜電晶體液晶顯示器模組 (thin film transistor liquid crystal display module) 的影像殘留 (image-sticking effect) 評估, 提出一個有效的光學量測方法, 其克服了傳統方法之不易使用儀器量測模組影像殘留的問題。

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.