快速響應、高對比、超輕薄之向列型液晶顯示元件

Abstract

本論文的主要研究目標在於探究一個液晶元件在顯示性能上面的極限。其中，我們特別針對向列型液晶元件在快速響應、高對比、輕薄化這些特徵上面做了深入的研究。 快速響應能力是實踐色序法驅動及時域分割之立體顯示技術的重要決定因素。而光學補償彎曲模態(optically compensated bend, OCB)是現階段響應速度最快的向列型液晶模態，因此，其被視為是最有可能用來實現上述之新穎顯示技術的液晶模態。不過，由於此液晶模態會有態間回復(inter-state recovery)的問題，使得應用上面困難重重。因此，在此論文中，我們提出利用水平對稱模態(symmetric H, Hs)以及高分子安定化之光學補償彎曲模態來避免態間回復的問題。採用水平對稱模態的難處在於其存在時間很短，因此，我們以一種搭配閃頻光源的錐光偏振儀來捕捉其暫態的液晶分子排列，並利用此結果，設計出一種高分子牆結構來延長其存在時間。藉由模擬顯示，利用這種設計結構，其存在時間可以延長十倍以上。 高對比是一個液晶顯示元件在表現生動影像時不可或缺的要素。在此論文中，我們提出兩個方法來提升影像對比。一個方法是將光學補償彎曲模態的液晶元件，藉由高分子反應，使其原來是暫態的鬆弛彎曲態(relaxed bend state)安定下來，由於鬆弛彎曲態具有高亮度的特性，因此，此安定化之後的液晶元件比傳統的高分子安定化元件於對比度方面至少提升1.5倍。另一個方法則是利用體散射層(volumetric scattering layer)來反射外界環境光，使炫光效應減少，因此，在日光下的對比度可以提升12倍。 輕量化與薄型化在行動顯示技術上面是極其重要的一環。我們在此提出將厚重的背光模組直接以輕薄的有機發光二極體(organic light emitting diode, OLED)取代。並且，更直接地將有機發光元件嵌入液晶顯示元件中，如此，其體積與重量將大幅減少。同時，此整合元件的有機發光二極體部分在暗態操作下可以將光源關掉，反觀現行的透反式(trans-flective)顯示器，在暗態下，其背光仍處於打開的狀態，因此，我們提出的整合元件同時可以在對比上大幅提昇、耗電量也可以減少許多。 本論文在科學上的貢獻在於提出一種利用閃頻光源來觀察甚至穩定本來只能暫時存在的液晶模態，此方法在液晶科學方面開創了一個新的研究領域。而在技術上，我們成功地實現了將有機發光元件嵌入液晶元件，並進一步地以體散射層來優化其光學性能，這在有機發光二極體的研究領域上是個重要的突破。最後，倘若整合所有本論文中得到的研究成果，我們將可以製作出一個具有高影像品質、低耗電、極輕薄且可撓式的新穎液晶顯示元件。

The main objective of this dissertation is to explore the ultimate features of a liquid crystal display (LCD) device. Particular attention is spent on the nematic type LCD device with the attributes of fast-switching, high-optical-contrast, and slim-form-factor. Fast-switching is a demanding requirement of an LCD device for realizing the novel display systems such as field sequential color LCD and temporally multiplexing 3D display. Regarding this requirement, pi-cell (also know as optically compensated bend, OCB) is the most promising LCD mode whose fast-switching results from its symmetric LC profile and parallel backflow. However, the issue of undesirable recovery hinders the pi-cell from commercial applications. In this thesis, we propose to use the symmetric H (Hs) state and the polymerized bend state to overcome this unwanted recovery. The difficulty of utilizing the Hs state results from its short lifetime. To resolve this issue, we propose a stroboscopic illumination assisted conoscopy to verify its transient LC profile. According to this result, we further design a polymer wall structure to extend its lifetime. With the verification of LCD simulation, the lifetime can be elongated by a factor of more than 10. High-optical-contrast is a critical attribute for an LCD device to show a vivid image. Here, we propose two methods for enhancing the optical contrast in the polymerized OCB-LCD and the organic light emitting diode (OLED) embedded LCD. In the polymerized OCB-LCD, we stabilize the relaxed bend state which has the highest brightness among the bend states. The optical contrast is enhanced by a factor of 1.5 compared with that of the conventionally polymerized OCB-LCD. In the OLED embedded LCD, after adopting the volumetric scattering layer (VSL), the optical contrast in the reflective mode is increased by a factor of 12. Slim-form-factor is the most important factor as considering the mobile display applications. We propose and successfully accomplish the experiment of embedding an OLED directly into an LCD device, which thus eliminate the bulky backlight module. Meanwhile, the contrast ratio can be significantly increased compared with the case of trans-flective display. Because the OLED section can be turned off, whereas the conventional trans-flective display should sustain the backlight to be turned on even in the dark state. Referring to the neology of “trans-flective” display, we therefore term this OLED embedded LCD as “emi-flective” display, which is a combination of “emissive” and “reflective.” Considering the scientific achievements, this dissertation demonstrates a stroboscopic illumination method, which opens a new era in observing and further stabilizing the transient states in an LCD device. As for the technical achievements, the realization of embedding the OLED into an LCD and the optical optimization with VSL have demonstrated a tremendous progress in OLED area. Combing these results, the integrated device is able to be low power consumption, high image quality, slim form factor, and fully flexible.