探討反置扭轉向列型液晶盒之預傾角量測與光電特性

Abstract

本論文主題是量測反置扭轉向列型液晶盒(inverse twisted nematic liquid crystal cell)的預傾角參數，並探討反置扭轉向列型液晶盒於不同預傾角參數且有無摻雜旋性分子下的光電特性。反置扭轉向列型模式在off-state時液晶分子為垂直排列，但在on-state時其液晶分子導軸則為扭轉排列結構。此種類似TN型模式的扭轉結構改善了穿透率對於不同波長敏感的問題，也因上下邊界液晶分子有光學性的互補，使得有如同VA型模式的傑出暗態表現。且反置扭轉向列型液晶盒會因預傾角參數的不同而改變其方向角錨定能(azimuthal anchoring energy) [1]，導致不同預傾角參數的液晶盒表現會傾向由雙折射效應(ECB mode)或旋光效應(twist mode)來主導 [2]。 論文中使用了一種簡單的光學量測方法來測定反置扭轉向列型液晶盒的預傾角，其方法是以增廣式瓊斯矩陣計算其理論再輔以晶體旋轉法量測。除此之外，我們也量測了預傾角參數在83°~89°間的反置扭轉向列型液晶盒之穿透度-電壓特性曲線與穿透度-時間特性曲線。從實驗中觀察到當施加高電壓於液晶盒時，預傾角越低(越遠離液晶盒法線)的樣品主要是由旋光效應來主導其光電特性，而預傾角越高(越靠近液晶盒法線)的樣品則由雙折射效應來主導。在動態反應的測量中，則發現當液晶盒由垂直態轉為扭轉態時會有光學反躍(反轉流動)的現象發生，嚴重地降低了液晶盒的反應速度。而我們在論文中也解釋了許多影響光學反躍現象的因素，例如：驅動電壓的大小、預傾角參數的高低、有無摻雜旋性分子等等。最後由實驗結果發現預傾角為83°且摻雜旋性分子的反置扭轉向列型液晶盒，其光學反躍的現象會比較輕微，而有較快的反應速度。 我們相信此量測反置扭轉向列型液晶盒之預傾角參數的方法具有相當的可靠度與精確度。且透過量測不同預傾角參數樣品的光電特性，讓我們理解預傾角對反置扭轉向列型液晶盒的影響性，有利於我們未來設計反置扭轉向列型液晶盒時，能夠挑選出最優化的參數來製作，使得液晶盒有更優異的光學表現。

In this thesis, we have measured the pretilt angles and investigated the electro-optical (E-O) properties of inverse twisted nematic (ITN) liquid crystal (LC) cells with or without chiral dopants. The ITN LC mode has a twisted-homeotropic structure in the quiescent state (off-state) and mixed modes of TN and birefringence in the turn-on state that improve the achromatic characteristic of transmittance-versus-voltage (T-V) curves. With a same pretilt angle below 〖10〗^° from cell normal and under crossed polarizers, the ITN mode has a darker off-state than the tilted-homeotropic mode. As mixed modes of twist and birefringence, the on-state E-O properties of the ITN cell depend on its azimuthal anchoring energy of LC alignments [2]. We can increase the azimuthal anchoring energy of an ITN LC cell by increasing the pretilt angle from cell normal [1]. We have used a simple optical method based on measurements of phase retardation versus out-of-plane cell rotation angle and data analyzed by extended Jones matrix method to obtain pretilt angles of ITN LC cell. In addition, we have also measured the E-O effects by controlling pretilt angles of ITN cells from 1^° to 7^° from cell normal by increasing the rubbing depth between rubbing cloth and LC-aligning polyimide (PI) film. We have found that the LC-director-twist effect of an ITN cell increased with increasing pretilt angle from cell normal, and opposite behavior for the LC-birefringent effect. In transient responses, we have observed that the optical-bounce (LC backflow) effect of the ITN LC cell occurred in transitions from homeotropic-to-twist states under high applied voltages. However, the occurrence of optical bounce slowed down the optical response. In this thesis, we have also studied the influences on the optical-bounce effect from the strength of electric field, the pretilt angle and chiral dopants. Finally, we have found that the ITN LC cell doped with chiral dopants and with a 7^° pretilt angle from cell normal had faster response times. In this thesis, we have revealed a simple, reliable and accurate method to measure pretilt angles of ITN LC cells. We have also reported the phenomenon that different pretilt angles had a significant impact on E-O properties of the ITN cell. We believe that our works reported here can help LCD engineers to optimize the important parameters of designing ITN LC cells to obtain optimized E-O effects for various display applications.