新型高對比、高亮度、無轉態之Pi-cell液晶顯示元件

Abstract

薄膜電晶体液晶顯示器(TFT-LCD)技術已取代傳統CRT電視應用的主流。為了使TFT-LCD顯示技術能夠穩佔龐大的TV應用市場商機，TFT-LCD開發、製造…等相關業者無不致力於持續改善TFT-LCD的影像品質。由於TFT-LCD先天上驅動原理的限制，因此TFT-LCD動畫顯示模糊效應(Motion blur effect)的問題一再地被廣泛討論並尋求解決的方法。在目前所有被應用的LCD技術中，光學補償彎曲模態(Optically Compensated Bend, OCB；另稱為Pi-cell)是目前反應速度最快的向列型液晶模態，由於液晶盒(LC-cell)特殊的結構設計使它不僅具有快速響應，同時也是具備廣視角顯示品質的液晶模態，因此在解決TFT-LCD TV動畫模糊效應上也是最強而有力的顯示模態。 由於Pi-cell快速響應的優點，使得高解析度色序法驅動的液晶顯示技術(Field Sequential Color Liquid Crystal Display)得以有實現的可能性。然而，Pi-cell有不同液晶形變態之間的轉換與回復(State transition and Recovery)問題，造成在實際應用上的困難，甚至有在顯示品質最佳化上的可能性存在。本論文中，針對此問題提出了二種新的解決方式： 一、以奈米結構將液晶配向層之表面進行改質，以便形成”成核點” (nucleus)，使得Pi-cell在由分散(Splay)模態轉換成彎曲(Bend)模態前必經的成核現象(Nucleation)及完成轉態所需時間由現在的2分鐘(cell gap~5μm,0→6V驅動)縮減至少於1秒。使得Pi-cell即便是需要經過轉態過程，亦能在幾乎無法察覺的時間內均勻地轉態完畢，且無需於瞬間外加約18 V高脈衝電壓。 二、在前一項研究中，並不能消除Pi-cell的轉態特性，在顯示器的電壓操作區間仍必須維持一個約2V的臨界電壓以保持彎曲(Bend)模態的穩定存在。基於液晶光學特性具有不同入射光波長的分散特性 (dispersion)，紅 (R)、綠(G)、藍(B) 三波長的電壓-透過率特性曲線(V-T curve)並不一致，因此RGB的臨界電壓並不相同 (VR< VG<VB)。為確保顯示器運作能維持在彎曲模態，必須將臨界電壓維持在VB，使得Pi-cell的透過率和對比相當程度被犧牲。因此，我們另外提出新穎的反應型單体(reactive monomer)應用及cell結構設計，使得Pi-cell中的液晶分子穩定在彎曲模態—驅動模態下，如此一來便克服了Pi-cell的轉態問題；同時，由於新結構的cell邊界殘留位相差值較小，亦使得新型RMM-Pi-cell在不加補償膜的情況下，其靜態對比度從傳統Pi-cell的26提昇到288 (實驗室測試樣品), 提昇度達11倍。 本論文在顯示應用技術領域上的貢獻在於提出以奈米結構對Pi-cell的液晶配向層進行改質，以改善其轉態時間及利用反應型單体層來消除現有Pi-cell本質的轉態特性，使得其所造成之光學品質缺點得以被解決或改善。而其實現方式僅需在現有的TFT-LCD的Cell製程架構中，增加1~2道簡單的製程，即可以完全相容於現有的製造方式，倘若將現有的顯示器技術結合本論文研究成果將可以創造出一種具有高影像品質的新型Pi-cell液晶顯示元件，使得此液晶模態的優勢不只有應用在一般TFT-LCD，更能實現色序法驅動的液晶顯示技術。

TFT-LCD Technology has become the mainstream in replacing CRTs in TV and other applications. In order for thin-film-transistor liquid-crystal display (TFT-LCD) technology to claim an even larger share of the TV market, manufacturers must continue to improve image quality. The issue of motion blur effect in TFT-LCD TVs has been widely discussed lately. Among the reported LC modes the Pi-cell, also known as Optically Compensated Bend (OCB) mode, has been found to be a strong candidate to reduce the motion blur effect. Due to the LC-cell structure and the driving scheme, Pi-cell is not only a wide viewing angle display technology, but also the fastest-response LCD mode among the commercialized LCD modes. Because of the fast response of the Pi-cell, the field sequential color LCD (FSC-LCD) which displays R, G and B colors in sequence in a pixel is promising for the high-resolution display. However, Pi-cell possesses intrinsic transition and recovery issues which lead to compromised optical properties, thus, limited in its applications. In this dissertation, we proposed two modified Pi-cells to resolve these issues. 1.Proposed Nanostructure Enhanced Pi-cell (NE-Pi-cell) modified the surface of alignment layer (PI) to create nuclei for speeding up the transition rate. The transition time of a NE-Pi-cell was reduced from 2 minutes to less than 1 sec compared with the conventional one. Moreover, the transition process was uniformly completed without applying high voltage pulse (~18V). 2.Even if the previous topic could speed up the transition rate to almost zero, the critical voltage also needed to be kept over 2V for maintaining the bend state of a Pi-cell. Based on the light dispersion property of liquid crystals, the critical voltage of R, G and B were different; i.e. VR< VG< VB. In order to confirm that the Pi-cell could operate in bend state, a critical voltage larger than VB needed to be chosen, which led to compromised optical qualities. Therefore, we suggested reactive monomer modified Pi-cell (RMM-Pi-cell) to eliminate the splay-to bend state transition. Besides, because of smaller residual retardation at the dark state, the static contrast ratio of a RMM-Pi-cell, compared with conventional Pi-cell, was improved from 26 to 288 (the test samples fabricated in laboratory), up to a factor of 11 without using compensation films. We have demonstrated a novel alignment layer modified method of a Pi-cell for uniform and fast transition without high voltage pulses. Moreover, a transition-free and high optical performance Pi-cell has also proposed in this dissertation. The proposed novel Pi-cells only need one or two simple extra processes in conventional manufacturing of TFT-LCD. Combining the research results with current high image quality LCD technology, the novel high image quality Pi-cell will be realized. The results can not only improve the image qualities for general TFT-LCD applications, but also realize the FSC-LCDs.