用三角波法量測液晶盒內離子濃度及其討論分析

Abstract

輸入三角電壓波量測液晶盒中離子總電荷量的概念，先後由日本東北大學的K. Ono先生和J. Nakanowatari先生以及Toyo公司的M. Inoue先生所提出，再利用量測之離子電荷密度的多寡來觀測其對於液晶顯示的影響。 本論文中之液晶盒皆為奇美提供之扭曲型液晶盒，我們設計一電路來放大量測的電流訊號，並利用類似的三角波量測法量測液晶盒中的離子電流。 我們在實驗中觀察到三角波法下所獲得之離子電流曲線和Toyo發表的文獻中所報導的結果有所不同，為了瞭解本論文量測到得離子電流機制，本論文引用楊界雄教授所發表的鐵電性液晶層中高、低解離率離子的概念，並且對楊教授發表公式稍做修改，修改成符合本實驗條件的公式，最後利用修改的公式來對照且分析本實驗中所觀測的離子電流物理機制。 利用上述物理模型能夠很清楚的解釋本論文中，不同溫度下三角波量測法所獲得的離子電流機制，並利用此物理模型來估算出扭曲型液晶盒中的高、低解離率離子的遷移率、不同溫度下低解離率離子的平衡濃度以及離子遷移時所需的活化能。 本論文也量測直接在ITO電極表面上做配向動作的扭曲型液晶盒，並用以照UV光與沒照UV光的做比較，發現照UV光後產生了許多高、低解離率的離子，不過照UV光後高、低解離率離子產生機制的研究，將不在本論文中討論。 另外利用Toyo機台作量測分析時，量測液晶盒中離子電荷量的方法，所使用的是操作者人為判斷來得到液晶盒中離子電荷密度的多寡。這樣的判別方式難免不夠客觀，為了客觀地分析數據，避免不同操作者主觀操作下所造成之誤差，本論文利用數學軟體Matlab，針對量測離子電荷密度寫了一套自動化的計算方式。

Triangular voltage waveforms have been used by Ono and Nakanowatari and Inoue to measure the ion currents of ferroelectric and nematic liquid crystal cells, respectively. In this thesis, we also used similar method with more sensitive circuits for low-level signals to measure ionic currents in TN test cells supplied by a TFT-LCD-panel-maker at different temperatures. Our measured results had substantial differences from that published in Inoue’s paper. In order to understand our experimental results, we used modified models of ionic generation and transport equations published for EPD x-ray imager and ferroelectric liquid crystal cells. We have compared our experimental results with numerical computations based on our new interpretation and obtained a good agreement between them. We used the facts that the measured ionic currents consisted of ions from both high-(HIR) and low-ionization-rate (LIR) impurities with different temporal behaviors for the determinations of ion concentration and mobility for HIR impurities and mobility, ion-recombination rate, and equilibrium ion concentration for LIR impurities within the TN cell. We have determined the ionizing activation energy of LIR impurities and also, for the first time, the mobility activation energy (in the range of 0.15 to 0.16 eV) for ions of LIR impurities within different TN cells. We have also made measurements on a TN cell aligned by rubbed ITO layers on glass substrates before and after subjected to uv-irradiation. The measured results showed negligibly small ionic currents prior to uv-irradiation. After uv-irradiation, the measured ionic currents increased by several orders of magnitude with comparable contributions from both uv-induced HIR and LIR impurities. Further investigation of observed phenomena is not a subject of this thesis. Furthermore, Inoue used intuitive manual base-line subtraction methods to obtain data of ionic current versus time to derive ion concentrations or ion densities (a sum of contributions from both HIR and LIR impurities). The results were subjected to variations from different manual operators so that they were useful only for relative comparisons and references. To remove subjective uncertainties, we have also developed a new numerical method implemented as Matlab programs to analyze our data for objective derivations of ionic currents with improved accuracy. We believe that our method of analysis can avoid unnecessary subjective errors as practiced from the teaching of Inoue’s paper or suggested by Toyo Corporation that has sold similar products to global LCD industries.