雙穩態膽固醇液晶可撓式顯示元件之光電特性研究

Abstract

為了發展軟性顯示元件，我們研究一種液晶顯示技術，稱為聚合物穩定型膽固醇液晶結構（Polymer-stabilized Cholesteric Texture,PSCT），此結構不需使用偏振片、濾色片與背光元件，加上此顯示技術具有雙穩態特性，因此可作為一個極低耗能且可撓性高的反射型顯示器。綜合上述優點，可廣泛應用於靜態型的電子書、電子紙之顯示元件。 我們摻雜高分子單體於螺距為可見光波長範圍之膽固醇液晶中，將其灌入水平配向之液晶盒裡，並照射紫外光聚合高分子單體，以增加結構穩定性。本論文主要研究高分子單體濃度及照光條件對反射頻譜、樣品對外加脈衝電壓響應及反應時間等光電特性之影響。實驗結果發現隨著高分子單體濃度或照光強度遞增，PSCT元件之對比度會漸減，但反應時間愈快。因此，我們發現摻雜高分子單體濃度為3wt%，照光條件為0.2mW/cm2(15hr)是PSCT元件最適合的參數。 由於單層結構PSCT元件只能單一反射左旋或右旋圓偏振光，所以最大理論反射率值為50%，對比度較低。因此提出双層結構PSCT元件來改善其反射率及對比度，而双層結構PSCT元件能同時反射左旋及右旋圓偏振光，所以最大理論反射率值為100%，使對比度也獲得提升。實驗結果得知，双層結構PSCT元件對比度相較單層結構PSCT元件提升至將近三倍之多。 最後，我們也成功將PSCT顯示技術由玻璃基板上轉移至軟性基板並製作出可撓式PSCT元件。

In order to develop flexible displays, we have studied a liquid crystal display mode with polymer-stabilized cholesteric texture, PSCT, that doesn't require polarizer, color filter or backlights. Therefore, the PSCT is low power consumption and attractive for bistable reflective display applications. In this study, we filled the mixture including a host nematic liquid crystal (MDA-003461), a chiral dopant (R-811), monomer (RM-257), and photo-initiator (DMPA) into anti-parallel cell. The cell was irradiated with UV intensity to polymerize the monomer, the PSCT with stable structure was formed. Then, we study on the electro-optic properties of PSCT samples with different ratio monomers and different irradiated UV light intensity. Experimental results show the contrast ratio decrease with higher monomer concentration or higher UV intensity, but the response time become faster. Therefore, we found three percent is the best monomer concentration and 0.2mW/cm2(15hr) is the best of UV intensity for those PSCT samples. Due to the typical single-layer PSCT structure can only reflect left- or right- circularly polarized light, the maximum of theoretical reflectance is 50% with low contrast. Therefore, we have used an anti-chiral double-layer PSCT structure to improve the reflectance and contrast. The double-layer PSCT structure reflects both left- and right- circularly polarized light, so the maximum reflectance can reach to 100% theoretically and the contrast ratio can also get promoted. The measured contrast ratio of the anti-chiral double-layer PSCT sample is nearly three times of that for a typical single-layer PSCT sample. Finally, we have also successfully transferred the PSCT display technology from glass substrates to flexible substrates.