灰階三：從 Planar state 形成

在有表面配向的情況下，當驅動電壓大於 Vth，液晶會先產生一些缺陷點，藉由缺 陷點的擴張進而轉變成 Focal conic state。圖 4.5.8 與圖 4.5.9 表分別以 20V 及 25V 驅動 至 Focal conic state，在此 Vth為 12V。當驅動電壓越大時，起始出現的缺陷點會更多且 分布均勻，藉由更多缺陷點擴張，達到 Focal conic state 的速度也會更快。以 20V 驅動，

完全達到 Focal conic state 需要約 30 秒；而以 25V 驅動僅需 15 秒。

圖 4.5.8 以 20V 驅動之狀態變化，驅動後(a) 3 秒 (b) 5 秒 (c) 8 秒

圖 4.5.9 以 25V 驅動之狀態變化，驅動後(a) 3 秒 (b) 4 秒 (c) 5 秒

由於缺陷點的產生，造成了 Planar 與 Focal conic state 共存的狀態，若以更高的電壓 做過飽和驅動（over driving），可以造成更多的缺陷點，驅動時間增加則會使缺陷點擴 大。利用這些特性，可以控制驅動電壓大小與時間來達到有配向情況下的灰階。為此，

實驗設計電壓驅動方式如圖 4.5.10。首先給予一高電壓 Vx做過飽和驅動 500 ms，此時 液晶盒表面將會出現許多缺陷點，之後隨即施加小電壓 7.5V，以穩定此時的液晶狀態。

(a) (b) (c)

(a) (b) (c)

圖 4.5.10 從 Planar state 達到灰階之電壓驅動設計

實驗中以 LCT-10-832_7.2μm 做為量測，Vx分別為 32V、32.6V 及 33.2V，驅動結果 如圖 4.5.11。當 V_x為 32V 時，瞬間產生的缺陷點與原有 Planar state 共存的狀態下，量

450 500 550 600 650

Planar

450 500 550 600 650

Planar

450 500 550 600 650

Planar

圖 4.5.12 驅動結果所對應色偏變化圖

4.5.5 結論

膽固醇液晶在有配向情況下，Planar state 具有高反射率，但驅動至 Focal conic state 後，反射率急遽下降，導致在電壓－色偏曲線上變化不連續，使灰階驅動有困難。本節 中提出 3 種方式，分別從 Focal conic state、Homeotropic state 及 Planar state 達成。其中 前兩種方式屬於被動驅動，需要依靠液晶本身自然回復而達成，因此回復速度取決於材 料特性，結果顯示需要達到數秒至數分鐘才能得到灰階。第三種屬於主動驅動，可控制 驅動電壓與時間，並且得到穩定灰階，較為可能做為實際應用。

0 10 20 30 40 50 60

0 2 4 6 8 10

0 10 20 30 40

22.0V 22.6V 23.2V V-Eab curve

Time (min) Voltage

Eab

Δ

第五章 結論

___________________________________________________________________________

本研究中對於介電異向性、黏滯係數與液晶盒間隙的改變探討其變因對於光電特性 的影響。介電異向性主要影響驅動電壓，其值越大，所需驅動電壓越小。黏滯係數主要 影響反應時間，特別是在 off-time 的部分，黏滯係數越大，反應時間越長。降低液晶盒 間隙更可以有效改善驅動電壓，實驗中所得最佳驅動結果，V_th= 5.5 V，V_sat= 9 V，V_reset= 28 V；但降低液晶盒間隙卻也同時造成反應時間的增加，間隙從 8μm 降低至 4μm，反 應時間從 50ms 增加至 300ms 左右，其原因可能是表面配向作用影響液晶自然回復至 Planar state 的趨勢，實驗中無表面配向的結果明顯較有表面配向的結果來的快。

在有表面配向的情況下，膽固醇液晶會得到將近理論極限的反射率，但缺點是配向 作用影響液晶在 Focal conic state 的穩態，以及 Planar 與 Focal conic state 的反射率差異 過大，形成灰階不連續的現象。為達到穩定灰階，必頇維持 Planar 與 Focal conic state 共存，並防止兩狀態互相影響，同時也要防止液晶與表面配向互相影響，而造成不穩定 的灰階。實驗中嘗詴三種方式來達到穩定灰階，分別從 Focal conic、Homeotropic 及 Planar state 來達成。前兩種為被動式驅動，所需時間較長，均在秒以上等級；最後一種為主動 式驅動，實驗中以過飽和驅動方式（over driving）驅動 500 ms，可得到最快速且最穩定 的灰階。

第六章 未來展望

___________________________________________________________________________

降低液晶盒間隙後，轉態過程受到表面效應的影響使得反應時間變長，但為了得 到低驅動電壓，降低間隙最為直接，效果也最好，因此詴圖改善反應時間為未來必定的 目標。目前的旋性分子參入液晶的量大約是 16 wt%左右，若採用高 HTP 的旋性分子，

混入向列型液晶的量可以更少，在黏滯性上可望更低。此外，高 HTP 也同時使得膽固 醇液晶的彈性常數 K₂₂更高，在回復的過程也會因此更快速，反應時間進而低，最好的 結果便是希望回復的彈性力遠大於表面作用力的影響，此時反應時間便不會有延遲的現 象產生。

在有配向的情況下，當驅動膽固醇液晶從 Planar state 至 Focal conic state 時，轉態 從缺陷點（defect）開始擴張至整個液晶盒。轉態的變化類似驅動 OCB 液晶時的結晶成 核（nucleation）現象，先前的研究顯示，含表面突起的配向層會改變表面的錨定能，使 得 OCB 再轉態時更容易進入到 bend mode [43]。同理，未來可將含有奈米粒子的配向層 表面應用於主動式穩態驅動。理論上膽固醇液晶的缺陷點會更容易出現，因此可以使用 更小的電壓或更短的時間驅動來達到穩態灰階。

圖 5.2.1 奈米粒子突起之配向層表面

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