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

• Main Authors: Tsu-Ruey Chou, 周祖銳
• Other Authors: 趙治宇
• Format: Others
• Language: en_US
• Published: 2017
• Online Access: http://ndltd.ncl.edu.tw/handle/64998720164011870412

博士 === 國立臺灣大學 === 物理學研究所 === 105 === 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.