Nanoimprint Formation of the Atomically Ultra-Smooth Surface of the PEDOT: PSS Conducting Polymer Thin Film for Flexible Optoelectronic Devices

Conductive polymer materials such as PEDOT: PSS with features of light weight and large area have become increasingly important in flexible electronics of organic thin-film solar cells or other optoelectronic devices [1]. In the construction of electronic devices on these polymer thin film electrodes, surface flatness and atomic- and/or nano-level surface control of conductive polymer thin films are essential for formation of fine and highly integrated devices. So far, by using the ultra-smooth sapphire (α-Al2O3 single crystal) wafers with self-organized approx. 0.3-nm high atomic-step straight nanopatterns as nanoimprinting molds [2], we had reported on the fabrication of surface ultra-smooth substrates by transferring their nanoscale patterns onto the surfaces of oxide glasses and thermoplastic polymers [3-5]. On the other hand, as for nanopatterning of PEDOT: PSS having no thermo-plasticity, there were only a few reports on stripe surface patterning (10nm depth and 700nm period) onto the thin film for increase of the organic photovoltaic device efficiency [6]. In this study, for formation of the atomically smooth surface on the PEDOT: PSS thin film, we investigated transcription of 0.3nm-high atomic-step and atomically flat terrace pattern of the sapphire mold onto the spin-coated PEDOT: PSS thin film surface via thermal nanoimprinting. We also characterized electric properties as well as the thermal or UV-light durability of the nanoimprinted pattern on the sample surfaces.<br/>A flexible transparent polyimide sheet (PI; t~20 µm, ECRIOS® VICT-Bnp, Mitsui Chemicals Co., Japan) was used as a polymer substrate for thin film deposition. PEDOT: PSS thin films were prepared by spin-coating PEDOT: PSS aqueous dispersion and drying at 170°C for 60 min. Thermal nanoimprinting was then performed at 200-270°C and 15 MPa for 40 min in vacuum on the PEDOT: PSS thin film surface using the sapphire mold with an atomic step and terrace width of about 600 nm. As a result, it was confirmed that an atomic step and terrace pattern corresponding to the sapphire mold could be transferred accurately onto the PEDOT: PSS thin film on the polyimide sheet in case of nanoimprinting at 260^oC. The successful transfer was achieved under conditions near the glass transition temperature (about 265°C) of the polyimide substrate, suggesting that the nanopattern transfer onto the PEDOT: PSS thin film might be related to softening phenomena of the substrate.<br/>[1] A. Radivo et al., RSC Adv., <b>4</b>, 34014 (2014).<br/>[2] M. Yoshimoto et al., Appl. Phys. Lett. <b>67</b>, 2615 (1995).<br/>[3] K. Shimada et al., Jpn. J. Appl. Phys.,<b> 55</b>, 098002 (2016).<br/>[4] G. Tan et al., Appl. Phys. Express., <b>7</b>, 055202 (2014).<br/>[5] Y. Miyake et al., Jpn. J. Appl. Phys., <b>50</b>, 078002 (2011).<br/>[6] J. B. Emah et al., Appl. Phys. Lett. <b>93</b>, 103301 (2008).