Measurements and Analyses of Ionic Impurities in Liquid Crystal Cells with Vertical Alignments
|關鍵字:||垂直配向液晶盒;離子現象;電壓保持率;相位差量測;Liquid Crystal Cells with Vertical Alignments;Ionic effect;Vhr;Phase|
For the first time, we study ionic phenomena of vertical aligned (VA) liquid crystal (LC) cells using polyimide (PI) alignment films by Voltage Holding Ratio (VHR) measurement. First, we measured heterodyne-interferometric phase versus applied voltages on the VA cells to confirm that the dominant ions were from the PI alignment layers (called EMR phenomenon) within the VA cells. We have derived our analytic equations based on published ionic generation and transport theory, and used these equations to fit the VHR data of the EMR samples measured at two different temperatures. As a surprise, there existed both ions from high-ionization–rate (HIR) and low-ionization–rate (LIR) impurities in PI alignment layers within the VA cells. Using the stated physical models, we have obtained HIR and LIR ion concentration and their corresponding mobility within PI alignment layers. However, the ion concentration of HIR impurities was almost independent of temperature as expected. Furthermore, we have also derived the corresponding activation energies of ion concentration for LIR impurities within PI layers from the data measured at two temperatures. The materials of alignment layer play an important role on the performances of TFT-LCDs. We do not clearly understand the origin of those ionic impurities within the alignment layer. With further investigations by changing LC and PI materials and by varying the cell process parameters, it might be possible to find out chemical compositions and structures of LIR and HIR ion within the PI alignment layers leading to reduce or eliminate them for the realization of high-quality and ion-free PI alignment layers for TFT-LCDs without image sticking. Our work has opened a new chapter to study ionic phenomena in PI alignment layers for VA cells.