Highly Conductive Pedot:Pss Film by Doping Ptsa and Post-Treatment with Dmso for ITO-Free Polymer Dispersed Liquid Crystal Cell
Tsu-Ruey Chou1*, Szu-Hua Chen1, Yen-Te Chiang2, Tien-Tzu Chang3, Chih-Wen Lin4, Chih-Yu Chao1,2
1Department of Physics, National Taiwan University, Taipei, Taiwan
2Institute of Applied Physics, National Taiwan University, Taipei, Taiwan
3Department of Computer Science and Information Engineering, National Taiwan University, Taipei, Taiwan
4Department of Electrical Engineering, California Institute of Technology, Pasadena, CA, USA
* presenting author:Tsu-Ruey Chou, email:trchou@phys.ntu.edu.tw
Polymer dispersed liquid crystal (PDLC) is an excellent candidate for manufacturing flexible display and smart window due to its high transmittance, wide viewing angle, and cell gap insensitivity. Traditionally, indium tin oxide (ITO) coated glass or plastic film were used as conducting substrates to control the scattering state of the PDLC film. However, the increasing cost and the fragile characteristic of ITO thin film make it inappropriate for fabricating large area or flexible devices. In order to construct a fully flexible device, many materials such as metal nanowire networks, thin metals, graphene, carbon nanotube, and conducting polymers have been studied as alternatives to replace ITO. Among these materials, poly(3,4-ethylenedioxythiophene):poly(styrensulfonate) (PEDOT:PSS) is one of the most potential materials due to its mechanical flexibility, high transparency in the visible range, and prospective potential for roll-to-roll manufacturing. [1] Nonetheless, the conductivity of the pristine PEDOT:PSS film, typically four orders of magnitude lower than ITO, restricts the applicability of PEDOT:PSS.

In this study, the doping and the post-treatment process were used in order to improve the conductivity of PEDOT:PSS. The conductivity of the spin coated PEDOT:PSS film enhanced greatly from 0.6 S/cm to 736 S/cm after 1.25 % of p-toluenesulfonic acid (PTSA, 50 wt%) was doped into the PEDOT:PSS aqueous dispersion. The post-treatment was performed by dropping a small amount of dimethyl sulfoxide on the dried PEDOT:PSS film, which further improved the conductivity to 1549 S/cm. The highly conductive PEDOT:PSS films were used as transparent electrode to fabricate ITO-free PDLC cell. The measured voltage-transmittance characteristics were similar to those of the sample fabricated by ITO coated glass. This work indicates that highly conductive PEDOT:PSS film is a promising material as transparent electrode to replace ITO for ITO-free and flexible PDLC devices.

[1] Yijie Xia, Kuan Sun, and Jianyong Ouyang, Solution-Processed Metallic Conducting Polymer Films as Transparent Electrode of Optoelectronic Devices. Adv. Mater. 24, 2436 (2012).
Author for Correspondence: cychao@phys.ntu.edu.tw