探討扭轉向列型液晶盒內液晶混合物與摩擦PI配向膜內的離子現象

Abstract

液晶盒內的離子濃度、遷移率和活化能是影響液晶顯示器顯示品質中有關灰度飄移、畫面閃爍、影像殘留、雲紋和製造良率之重要參數。在本論文中，我們首先利用相位差(Phase Difference)量測方法來判斷在不同溫度下液晶盒中的離子行為主要為液晶層內離子主導(Depressed Molecular Reorientation，DMR）或摩擦PI配向層內離子所主導(Enhanced Molecular Reorientation，EMR)，接著再由本論文所提出的新方法來分析用摩擦PI配向膜的扭轉向列型液晶盒中在不同溫度下所量測到的電壓保持率（Voltage Holding Ratio，VHR），來推導在不同溫度下來自液晶混合物及配向膜材料的高解離率（High-Ionization-Rate，HIR）和低解離率雜質離子（Low-Ionization-Rate，LIR）的初始雜質離子濃度、遷移率和解離活化能及來自液晶混合物材料的低解離率雜質離子遷移率活化能。 本論文結果証明，在顯示出DMR 現象的TN 液晶盒中，主要離子是 由該盒中液晶層高、低解離率離子所主導，與最近發表的文獻所宣稱的， 由低解離率離子所主導，並不一致。而本論文也在顯示出EMR 現象的TN 液晶盒中，首次證實了雜質離子存在於該液晶盒中摩擦PI 配向層內，在 該配向層裡，同時存在著高、低解離率的雜質離子主導影響中間液晶層的 光電效應。本論文用理論所推導的雜質離子的解離和運動方程式，來分析 由實驗所量測的數據，從而獲得不同雜質離子種類於不同溫度下的離子濃 度、遷移率及其所對應的解離與遷移率的活化能。本論文也探討，在外加 電壓5V 下的DMR TN 液晶盒中，也存在著高、低解離率的雜質離子， 但所推導的離子物理參數與在外加電壓為1V 時相當不同。 目前我們並不清楚這些存在於摩擦PI 配向層中的高、低解離率的雜質 的來源，但我們認為利用本論文所揭示的實驗和解析方法，結合材料的不 同成份的調配以及製程參數的改變，在進一步探討之下，有可能驗證這些 高、低解離率的雜質離子的來源及其化學成份和結構，有助於研發無高、 低解離率雜質離子的高品質摩擦PI 配向膜和液晶混合物，以達成無畫面 閃爍與影像殘留的低耗電的高顯示品質液晶顯示器。

Ion concentration, mobility and activation energy in liquid crystal cells are three most important parameters to affect the display qualities of LCDs in terms of threshold voltage, gray-level shift, charge holding ratio, flickers, image-sticking, and Mura. In this thesis, we, first, measured the phase relaxation of a TN cell under low-frequency voltage square wave at an amplitude equal to the middle gray level of the TN sample aligned by rubbed polyimide (PI) films at different temperatures to find out if the dominant ionic species are from the LC mixture (DMR, Depressed Molecular Reorientation) or from the PI layers (EMR, Enhanced Molecular Reorientation). Then, we used our newly developed methods to measure and analyze the data of voltage-holding-ratios (VHRs) of DMR and EMR TN cells at different temperatures to determine the ion concentration, the mobility and the activation energies of ion-dissociation rate of high-ionization-rate (HIR) and low-ionization-rate (LIR) impurities where the dominant ions were either from the LC mixture or PI layers of the TN cell. In the case that dominant ions were from the LC mixture of the TN cell, we have also determined the activation energies of ionic mobility of LIR impurities. Prior publications on the measurement of ion concentration and mobility did not distinguish ions either from HIR or LIR impurities. Our group have developed experimental and analytic methods to determine both mobility and ion concentration of HIR and LIR impurities in TN LCDs aligned by rubbed polyimide film, and found out recently that the dominant ionic species in LC mixture in a DMR sample came from impurities of low-ionization-rate(LIR). However, for the first time, we have also observed that both ions from HIR and LIR impurities existed in different DMR TN samples. We have also carried out the investigation for the case when an applied voltage of 5 V was applied to the DMR TN sample to orient the directors of LC molecules parallel to the applied electric field. However, the derived physical parameters on ions were quite different from those when 1 V was applied to the TN cell. In this thesis, we have observed and confirmed that ions from HIR and LIR impurities also existed in the rubbed PI alignment layers of TN cells exhibiting EMR behavior. For the first time in the world, we have successfully extended our methods of measurements and analyses to solve the EMR cases where the dominant ions are from the polyimide alignment layers of the TN cell. We have no knowledge where theses HIR and LIR impurities in the rubbed PI films came from. But we believe that, by applying our experimental and analytical methods reported in this thesis to investigate the TN cells made of LC mixtures of different concentration of key components and/or different PI material and process, we may find out the origins and the chemical structures of ions from HIR and LIR impurities existing in these EMR and DMR TN samples. The results of further investigation will lead us to develop PI alignment layers and LC mixtures without HIR and LIR impurities, and ultimately, the realization of low-power-consumption, high-display-quality TFT-LCDs free of flickers and image-sticking.