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 . In this trend, AgNW-based transparent electrodes are expected as an alternative material to indium-tin-oxide (ITO)  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)  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  for large-area and wearable optoelectronic applications.
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