Ashuya Takemoto1 2 3 Teppei Araki1 2 Yuki Noda1 Shusuke Yoshimoto1 Takafumi Uemura1 Tsuyoshi Sekitani1 2

1, Osaka University, Ibaraki, , Japan
2, Osaka University, Suita, , Japan
3, National Institute of Advanced Industrial Science and Technology (AIST), Suita, , Japan

We report a high-resolution printing technique of Ag-nanowire (AgNW) -based electrodes on 1-µm-thick polymer films, which enabled the fabrication of transparent flexible organic thin-film transistors (OTFTs). All electrodes in the OTFTs were fabricated with this technique, where alcohol-based AgNW dispersion was applied on hydrophilic/hydrophobic-patterned surface. The processing temperature below 120°C allows the device integration with ultrathin (~1 µm) and transparent organic materials. Thus, the OTFTs exhibited mechanical stability under a bending radii of ~1 mm (~0.6% strain) and a visible transmittance around 80%.
Flexible transparent electronics will pave a way for novel optoelectronic applications such as paper-like displays, flexible touch panels and smart contact lens [1]. In this trend, AgNW-based transparent electrodes are expected as an alternative material to indium-tin-oxide (ITO) [1] owing to their mechanical flexibility, high conductivity and optical transparency. In addition, they can be fabricated with printing techniques such as ink jet, gravure, and screen printing, whose low-temperature processabilities can facilitate device integration with flexible polymer materials. However, printing AgNW-based electrodes with widths/spacings of less than 100 µm remains challenging.
The present work demonstrates the feasibility of printing AgNW-based electrodes with widths/spacings down to 20 µm in high accuracy of less than 3 µm, contributing to flexible transparent electronics. The AgNW-based electrodes exhibited bending stability to 1% strain. Capitalizing on such electrodes, we developed the transparent flexible OTFTs. First, AgNW-based electrodes with 50 µm width were patterned as the source/drain electrodes with channel lengths down to 25 µm on a 1-µm-thick film of poly(p-xylylene) (parylene). To form the active layer, 30-nm-thick layers of a benzothienobenzothiophene derivative (C8-BTBT) [2] was deposited on the source/drain electrodes. After that, a parylene layer with ~400 nm thickness was deposited as the gate dielectric. The AgNW-based gate electrodes were patterned on top of the gate dielectric. Finally, a 1-µm-thick encapsulation layer of parylene was deposited. The completed OTFTs exhibited an on/off ratio of ~106 and a hole mobility of 0.4 cm2V-1s-1. Furthermore, they had a high visible transmittance around 80% and withstood a bending radii of ~1 mm. Due to these versatile performances, it is implied that the OTFTs based on printed AgNW-based electrodes can serve as an alternative device to recent p-channel transparent TFTs with oxide semiconductors [3] for large-area and wearable optoelectronic applications.
[1] T. Sannicolo et al., Small 12, 6052 (2016).
[2] Y. Yuan et al., Nat. Commun. 5, 3005 (2014).
[3] Z. Wang et al., Adv. Mater. 28, 3831 (2016).