液體材料對液晶顯示器之光電特性的改善

Enhancement on Electro-Optical Properties of Liquid Crystal Displays by Modifications from Liquid Materials

江衍德(Yen-Te Chiang)

指導教授 ： 趙治宇

國立臺灣大學/理學院/應用物理研究所/博士(2018年)

https://doi.org/10.6342/NTU201800176

摘要

近年來，液晶顯示的市場持續擴張，各種不同的驅動模組相應而生以應對各種不同的應用裝置需求，且對光電特性的要求也越來越高。在本論文中，我們成功利用液體材料的性質，改善了包括傳統扭旋型液晶(TN mode)、反式扭旋型液晶(ITN mode)、及高分子散射型液晶(PDLC mode)等多種不同液晶驅動模式的光電特性，同時也針對不同驅動模式，找出液體改善光電特性的原因和機制，期許能在未來應用更多不同種類的液體來提高液晶顯示的效能。
針對扭旋型液晶和反式扭旋型液晶，我們嘗試了液體摻雜來取代常見的奈米粒子摻雜，奈米粒子雖然已經有大量文獻表明他們能有效的改善液晶的光電特性，但卻有沉澱和粒子互相吸附堆積的問題，容易降低液晶顯示的穩定性或破壞液晶的相位，而使用液體就能避免這方面的缺陷。我們選擇了甲苯和1-甲基萘這兩種芳香烴液體並將其摻雜進扭旋型液晶當中，我們發現這兩種液體都能大幅降低液晶的旋轉黏滯性，因此改善扭旋型液晶的光電特性，且甲苯的表現比甲基萘的影響要更為顯著，在液晶中摻雜10 wt%的甲苯就能同時降低液晶的驅動電壓和加快反應時間達18%，因此我們選擇甲苯進一步摻雜進反式扭旋型液晶中觀察其對光電特性的影響。在反式扭旋系統中我們觀察到液晶旋轉黏滯性的降低是源自於甲苯分子能穿透於兩兩液晶分子中間，增加液晶分子間的距離，降低其分子作用力，最終在巨觀表現上改變了介電異向性、旋轉黏滯性及黏彈係數等液晶的物理性質，最終甲苯也成功降低了驅動電壓1.1伏特並讓總反應時間加快了49%。
針對高分子散射型液晶，我們選擇了液體的高分子單體EDOT作為原料，利用電聚合的方式讓聚合後的高分子與液晶產生相分離來製作高分子散射型液晶。以此方法聚合而成的高分子PEDOT是一種導電高分子，因為其能傳導電子的特性，使得這種高分子散射液晶具有相較於傳統高分子散射液晶極低的閾值和驅動電壓(分別約3伏特和5伏特)，雖然我們發現在此系統下，高分子若沒有足夠的聚合時間，就會讓穿透率隨電壓的上升而出現震盪的現象，但只要找到合適聚合時間讓高分子均勻散布在整片液晶元件中，穿透率變化就能趨於正常。
本論文針對三種液晶驅動模式進行改良，都獲得顯著的光電特性提升，足以讓我們在發展液晶顯示應用的過程中提高對液體材料的關注，希望在之後能有更多研究將液體的多樣性應用在液晶領域當中。

關鍵字

液晶 ； 甲苯 ； 1-甲基萘 ； EDOT ； 光電特性 ； 液體摻雜 ； 電聚合

並列摘要

In recent years, the development of liquid crystal display (LCD) has greatly extended because of the flourishing growth of the mobile devices industry. A variety of liquid crystal (LC) driving modes have been improved to meet the market requirements. In this dissertation, we successfully enhance the electro-optical (EO) properties of various LC driving modes, including twisted nematic (TN) mode, inverse twisted nematic (ITN) mode, and polymer dispersed liquid crystal (PDLC) mode by using liquid materials and investigate the physical properties of LCs modified by these liquid materials to characterize their advantages in the future position for LCD devices.
For the improvement of TN mode and ITN mode, we chose aromatic hydrocarbon (AH) liquid as dopants to avoid the possible defects caused by nanoparticles, which are the common dopants chosen by previous studies. Toluene and 1-methylnaphthalene were effectively lowered the driving voltage and response time of the TN LCs. An 18% decrease in both driving voltage and response time was achieved by doping 10 wt% toluene into LCs, which showed stronger enhancements than 1-methylnaphthalene of the same concentration. This enhancement is due to a large amount of reduction in the rotational viscosity of AH liquids doped LCs. Toluene was then further doped into ITN LCs to enhance the EO properties. Driving voltage was 1.1 V lower and the total response time was reduced by 49% after toluene doped into LCs. In this case, we first provided the verification that the enhancements of EO properties was due to the penetration by toluene molecules between LC molecules. Hence, the dielectric anisotropy and the rotational viscosity as well as the visco-elastic coefficient of LCs are changed to present enhanced EO properties.
For the improvement of PDLC mode, we chose a liquid monomer 3,4-ethylenedioxythiophene (EDOT) to form polymer matrix through polymerization induced phase separation (PIPS) method. Conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) was polymerized from electro-polymerization process. Under this fabrication process, some transmittance vibrations at low voltage were discovered, but they can be well-suppressed under suitable polymerization time to achieve smooth transmittance-voltage curve. As a result, the obtained PEDOT-based PDLC exhibited low threshold and driving voltage (around 3 V and 5 V, respectively) compared with the traditional PDLC systems.
All these works showed remarkable enhancements through the modifications from liquid materials. These results provide a promising vision to optimize a wide range of LCD systems.

並列關鍵字

liquid crystals ； toluene ； 1-methylnaphthalene ； EDOT ； electro-optical properties ； liquid doping ； electro-polymerization

參考文獻

Chapter 1

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