利用摻雜及後處理技術製作高導電PEDOT:PSS膜用於液晶顯示器

Highly Conductive PEDOT:PSS Film Produced by Doping and Post-treatment Method for ITO-Free Liquid Crystal Displays

周祖銳(Tsu-Ruey Chou)

指導教授 ： 趙治宇

國立臺灣大學/理學院/物理學研究所/博士(2017年)

https://doi.org/10.6342/NTU201703356

摘要

液晶顯示器發展至今已超過三十年，是目前市場上最主流的顯示器。日常生活中所使用的智慧型手機、平板電腦、大尺寸平面電視等產品多數選用液晶顯示器做為螢幕。為因應下一個世代的光電元件之需求，可撓式液晶元件是未來發展的方向。因此，使用氧化銦錫(ITO)做為透明電極的傳統方法必須要有所改變。根據過去的研究文獻，奈米金屬線、石墨烯、印刷式網格狀金屬、導電高分子等材料均有取代ITO成為透明電極的可能性。其中，導電高分子PEDOT:PSS由於具有極佳的可撓性、良好的熱穩定性以及可見光波段的高穿透率，被認為是相當有潛力的材料之一。此外，PEDOT:PSS可以分散在水中的特性，使其非常適合用在低成本的溶液製程上。然而PEDOT:PSS的導電度較商品化的ITO低了四個數量級，相當程度地限制了PEDOT:PSS應用性。本論文中，我們使用了兩種方法改善PEDOT:PSS的導電度。此外，我們利用提升過導電度的PEDOT:PSS膜做為透明電極，製做並分析了兩種不同類型的液晶元件。
第三章中，我們利用二甲基亞碸(DMSO)對PEDOT:PSS膜進行後處理，成功將其導電度從0.7 S/cm大幅提升至1185 S/cm。不僅如此，當我們將提升導電度的PEDOT:PSS膜做磨擦配向處理後，PEDOT:PSS膜可以均勻地水平排列液晶分子。如此一來，PEDOT:PSS膜不但可以做為透明電極，還能做為液晶盒的配向層，同時兼具兩種功能。我們利用磨擦配向處理過的PEDOT:PSS膜，首次成功地製做出90度扭旋向列型液晶盒。量測結果顯示該液晶盒之光電特性，不但跟傳統上利用ITO搭配聚醯胺做出來的液晶盒之光電特性是可相比的，其穩定度也達到三個月以上。
第四章中，我們使用摻雜以及後處理兩種技術來改善PEDOT:PSS的導電度。當PEDOT:PSS水性分散液中摻雜1.25% (v/v)之對甲苯磺酸(PTSA)水溶液後，自旋塗佈在基板上的PEDOT:PSS膜，其導電度可從0.7 S/cm提高至765 S/cm。接著我們利用DMSO對摻雜PTSA之PEDOT:PSS膜做後處理，導電度可進一步達到1459 S/cm。在此研究中，我們將高導電的PEDOT:PSS膜做為高分子分散型液晶盒之透明電極。量測結果顯示我們製做出來的樣品其光電特性是可與利用ITO製做出來的樣品相比的。
本論文中的兩個研究，均顯示高導電的PEDOT:PSS膜擁有相當大的潛力取代ITO作為液晶盒中的透明電極。未來人類發展新型態的光電元件時，我們相信PEDOT:PSS是一大有可為材料。

關鍵字

導電高分子 ； PEDOT:PSS ； 後處理 ； 摻雜 ； 扭旋向列型液晶 ； 高分子分散型液晶

並列摘要

Liquid crystal displays (LCDs) have been developed over 30 years and have become the most dominant product in the display market. Nowadays, LCDs are widely used in our daily life, e.g., smart phones, tablet PCs, and large panel televisions. In order to develop flexible liquid crystal (LC) cells for next generation optoelectronic devices, several potential materials such as metal nanowires, graphene, printable metal grids, and conducting polymers have been investigated as alternatives to replace the traditional transparent electrode, which is composed of indium tin oxide (ITO). In particular, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is one of the most potential materials due to its excellent mechanical flexibility, good thermal stability, and high transparency in the visible range. In addition, the water dispersibility of PEDOT:PSS makes it very suitable for low-cost solution processing method. However, the conductivity of the pristine PEDOT:PSS film, typically four orders of magnitude lower than that of the commercial ITO, restricts the applicability of PEDOT:PSS. In this dissertation, we demonstrate two methods to enhance the conductivity of the PEDOT:PSS film. Furthermore, ITO-free LC devices employing the conductivity improved PEDOT:PSS film as transparent electrode were fabricated and investigated.
In Chapter 3, a simple and effective method to enhance the conductivity of the PEDOT:PSS film from 0.7 to 1185 S/cm by post-treatment with dimethyl sulfoxide (DMSO) is illustrated. After the rubbing technique was applied, the DMSO-treated PEDOT:PSS film revealed uniform planar alignment which could be used as both the transparent electrode and the alignment layer in the LC device. In this study, ITO-free twisted nematic (TN) LC cells fabricated by the rubbed PEDOT:PSS film were made for the first time, and they show comparable electro-optical properties relative to TN LC cells constructed by ITO and polyimide. Moreover, the reliability of the ITO-free TN LC samples are at least three months.
In Chapter 4, doping technique and the post-treatment process were both employed to enhance the conductivity of PEDOT:PSS. The conductivity of the spin coated PEDOT:PSS film increased greatly from 0.7 to 736 S/cm after 1.25% (v/v) of p-toluenesulfonic acid (PTSA) solution was added into the PEDOT:PSS aqueous dispersion. The conductivity of the PTSA doped PEDOT:PSS film further enhanced to 1549 S/cm by post-treatment with DMSO. In this study, the highly conductive PTSA doped PEDOT:PSS film was utilized as the transparent electrode to manufacture ITO-free polymer dispersed liquid crystal (PDLC) cells. The measured electro-optical characteristics of the ITO-free PDLC cells were comparable to those of the PDLC cells assembled by the ITO glass. This study reveals that the highly conductive PEDOT:PSS film is an excellent candidate for ITO-free PDLC devices.
As these two studies show the potential and facility of the highly conductive PEDOT:PSS film to be used as transparent electrode for ITO-free LC cells, we believe that PEDOT:PSS is a promising material for industry to manufacture optoelectronic devices in the future.

並列關鍵字

conducting polymer ； PEDOT:PSS ； post-treatment ； doping ； twisted nematic liquid crystal ； polymer dispersed liquid crystal

參考文獻

6. 黃久菖，以液晶元件作為光學調制器並實現其增強光電性能上的應用 (國立臺灣大學物理系研究所博士論文，台北，2015年)。

4. 黃久菖，以液晶元件作為光學調制器並實現其增強光電性能上的應用 (國立臺灣大學物理系研究所博士論文，台北，2015年)。

1. P. Yeh and C. Gu, Optics of Liquid Crystal Displays (John Wiley & Sons, Inc., USA, 1999).

3. G. Stojmenovik, Ion Transport and Boundary Image Retention in Nematic Liquid Crystal Displays (Universiteit Gent, Gent, 2004).

5. 陳培翔，奈米元件之製作、應用與其物理性質之研究 (國立臺灣大學物理系研究所博士論文，台北，2009年)。