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http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/21986
Title: | 低功率邊緣場效驅動液晶顯示器配向角與電極結構之閃爍問題模擬研究 Effects of Rubbing Angle and Electrode Structure on Flicker in Low-Power-Consumption Fringe-Field-Switching Liquid Crystal Display |
Authors: | Jhih-Wei Jhang 張志偉 |
Advisor: | 蔡永傑(Wing-Kit Choi) |
Keyword: | 閃爍,低功消耗,邊緣場效驅動顯示器,配向角,正型液晶, flicker,low-power consumption,fringe-field-switching(FFS),rubbing angle,positive dielectric liquid crystal, |
Publication Year : | 2018 |
Degree: | 碩士 |
Abstract: | 日常生活中電子化裝置消耗大量的能源,為了減少全球能源消耗,因此降低電子裝置的消耗是很重要的議題,我們可以從日常生活中常見的手機、平板以及電視去做改善。
為了降低顯示器的功率消耗,最顯而易見的就是從背光源去下手,除此之外,可以利用降低驅動電壓的頻率,因為消耗功率正比於驅動電壓頻率,雖然高頻顯示器擁有高表現的動態影像,但是靜態影像在顯示器發展上也是相當重要的,低頻驅動模式也是需要關注的議題。 現今有兩個主流的水平排列液晶顯示器:平面液晶顯示器(IPS)與邊緣場效液晶顯示器(FFS),都有在物質兩端的夾層結構,且在一端有交錯電極分布。IPS結構為同層的畫素電極和共同電極交錯,電場為對稱分布,使液晶可以有相同的旋轉週期特性;FFS結構為同層的畫素電極交錯,下有絕緣層去隔離共同電極,因此會有非對稱的電場分布,但電場週期只有IPS的一半,可以使整個電極區的液晶發生旋轉,因此會有較高的亮度。 FFS已經主導液晶顯示器市場,特別是可攜式裝置,因為它有高開口率、高亮度、低操作電壓、廣視角以及良好的觸控面板耐受性。而對於低功率消耗的需求,可以利用低頻驅動於靜態影像,我們希望在靜態影像時轉換成低頻驅動來降低功率消耗,因此討論到FFS的低頻驅動,但是此時影像品質會出現嚴重的影像閃爍,因為液晶有彎電效應(Flexoelectric effect),彎電效應是一個自發偶極矩受於彈性力和液晶的電極化,於低頻時,液晶在正、負偏壓的施加電壓時間中,液晶有足夠時間能夠旋轉並產生彎曲,會使在正、負偏壓中呈現不同的穿透度表現,因此產生影像閃爍(Flicker)現象。 最近五年大家開始研究這個議題,主要有三個方向,首先,透過改變液晶物理特性,使黏滯性(Viscosity)減少、降低正型液晶的各向異性介電常數(Dielectric anisotropy)、採用負型液晶、降低彎電特性以及減少彈性係數(Elastic constant)皆可降低閃爍。第二,透過改變電極結構,電極的寬度和間距會影響到電場分布及強度,再搭配調整絕緣層的厚度也可以降低閃爍。第三,施加雙極化電壓(Bipolar voltage),可以使電場均勻對稱分布降低閃爍發生。 本篇研究正型液晶的低功率邊緣場效驅動顯示器配向角對閃爍的影響,首先發現靜態閃爍隨配向角的增加,會先下降再上升的趨勢,因此會有最低閃爍的配向角情形,在本篇了解配向角影響靜態閃爍的原因後,第二點發現液晶的彎電係數:彎曲狀(Bend)與散狀(Splay)的彎電係數(Flexoelectric coefficient),兩者會影響到最低閃爍時的配向角以及閃爍受配向角影響的幅度,當彎曲狀彎電係數越大,發生最低閃爍的配向角越低;而當散狀彎電係數越大,靜態閃爍(Static image-flickering)隨配向角變化增大,配向角影響閃爍程度更大,因此取適當的配向角更加重要。接下來因電極結構改變會影響電場分佈,因此經結構測試,實驗結果得知結構上的電極寬度與間距影響靜態閃爍與配向角關係,最低閃爍的配向角會受到影響,主要受於彎電係數與結構上比例的影響,造成傾角變化而影響到穿透度,配向角會影響液晶的旋轉,使傾角差異更大則閃爍會增大,傾角差異降低則閃爍會減小的現象。最後我們再觀測動態閃爍與配向角間的關係,動態閃爍(Dynamic image-flickering)也就是光學突波(Optical bounce),發現光學突波與配向角無明顯趨勢,以及幅度變動極小,所以總體來說,配向角會影響靜態閃爍但不影響動態閃爍,閃爍程度受配向角影響確實能有最低值,固定電極結構下,我們可以透過液晶的彎電係數選用合適的配向角來降低閃爍,若彎曲狀彎電係數極大,可以採用5度的配向角,若彎曲狀彎電係數極小,則採用15度的配向角,可以有較佳的光學表現。 一般配向角為5至15度,配向角升高會使穿透率下降、操作電壓上升,但是響應時間會加快,在我們研究又發現會影響閃爍情形,本篇材料MLC-0648的使用,在於一般配向角,差異到超過40%的閃爍狀況,其配向角的取捨確實有所影響,並且可以在過去研究上,控制液晶參數及改變電極結構等方法下,更進一步透過配向角改善影向閃爍情形,使影像閃爍情形降至最低。 Electronic devices consume large amount of energy in daily life. For reducing global energy consumption, low power consumption is an important issue. We can improve from common cell phones and TVs. Low-frequency driving of a LCD panel to reduce power consumption has been followed with interest recently. It can use on static images through changing frequency of driving. However, an unacceptable performance, image flickering, occurs due to flexoelectric effect. Nowadays, there are two mainstream homogeneous aligned liquid crystal display: in-plane-switching and fringe-field-switching LCD. Fringe-field-switching LCD has dominated market of portable device. It has high aperture ratio, high transmittance, low operative voltage, wide viewing angle and better touch panel tolerance. Because the pixel and common electrode are not the same layer in fringe-field-switching structure, they can lead to asymmetric electric field and increase image flickering. There are many methods reducing image flickering in a FFS cell to be proposed. First, such as controlling the physical characteristics of LC by means of dielectric anisotropy, elastic constant, viscosity, and flexoelectric anisotropy. The tilt angle of low-dielectric-anisotropy LC is less sensitive to the applied electric field, so low dielectric anisotropy can help suppress the image flicker. High elastic constant makes LC difficult to bend, and it affects the time between splay and bend state. The viscosity is related to response time, so it also affects the rotation time of LC between neighbouring frames. The image flicker induced by the flexoelectric effect is related to flexoelectric anisotropy, and decreasing the difference between splay and bend flexoelectric coefficient can reduce the image flicker. Second, change the electrode structure. Third, some studies also utilize bipolar driving method. These two methods both make the electric field distribution symmetrize during positive and negative frames. Therefore, they have the same transmittance during neighbouring frames. In the study, we investigated the effect of rubbing angle on the image flickering phenomenon in a fringe-field-switching liquid crystal cell with positive dielectric LC. Through the research of static image flickering, we get flicker to decrease first and then increase as rubbing angle is higher. We continue to study factors which affecting the result. We test for different flexoelectric coefficient, electrode structure, and optical bounce phenomenon. Finally, we find flexoelectric of splay and bend. Flexoelectric of splay can mainly make the range of flicker change as the rubbing angle increases. When flexoelectric of splay is bigger, the rubbing angle can affect flicker more. Flexoelectric of bend can induce rubbing angle of the lowest flicker to change. When flexoelectric of bend is bigger, the rubbing angle of the lowest flicker is decreasing. We find that selecting an appropriate rubbing angle to achieve better optical performance through knowing flexoelectric coefficient. In general, the rubbing angle is between 5 and 15 degrees in FFS cells. When we increase rubbing angle, the transmittance decreases and operative voltage increases, but it makes response time faster. In our study, the rubbing angle also affects the image flicker on display. Therefore, we select our demands to apply appropriate rubbing angle and achieve required optical performance. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/21986 |
DOI: | 10.6342/NTU201803491 |
Fulltext Rights: | 未授權 |
Appears in Collections: | 光電工程學研究所 |
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ntu-107-1.pdf Restricted Access | 4.53 MB | Adobe PDF |
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