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Zhi Jiang1 2 Kenjiro Fukuda1 3 4 Sungjun Park3 Takao Someya1 2 3

1, Thin-Film Device Laboratory, RIKEN, Hirosawa, Wako, Saitama, , Japan
2, Electrical and Electronic Engineering and Information Systems, The University of Tokyo, Tokyo, , Japan
3, Center for Emergent Matter Science, RIKEN, Saitama, , Japan
4, Japan Science and Technology Agency, Saitama, , Japan

Conformable large-area optoelectronic devices are necessary for self-powered ultra-flexible applications to realize multi-functional systems, such as sensing, imaging, and healthcare-monitoring [1, 2]. Since conventional transparent electrodes, such as ITO, are brittle, novel transparent conductors owing good mechanical robustness are indispensable to these ultra-flexible photonic devices. Printing technology will make these novel ultra-flexible transparent electrodes closer to the step of their real application [3]; however, it remains challenging to obtain feasible printed transparent electrodes with ultra-thinness, good uniformity over a large area, and high conductance simultaneously [4]. There exists a trade-off between thickness and conductance. Highly conductive Ag mesh electrodes usually have a thickness in the range of 2-20 μm, which can limit the flexibility and inhibit utilization in ultrathin photonic devices. Reducing the thickness and keeping a high conductivity simultaneously is needed for ultra-flexible photonic devices.

Here, we present ultra-flexible and mechanically durable Ag mesh transparent electrodes fabricated by a reverse offset printing technique, which simultaneously achieved high conductance, high transparency, and good mechanical properties. Reverse-offset printing technology enabled high resolution (100 nm and 5 μm in thickness and width, respectively) and high uniformity of Ag mesh over a large area. The high uniformity comes from the good quality of Ag nanoparticle ink (good dispersion, ultrafine diameter, and uniformity of Ag nanoparticles), and small roughness of both glass cliché and PDMS transfer blanket. Consequently, the printed transparent electrodes exhibited a 17 Ω/sq sheet resistance at 93.2% transmittance. Furthermore, they showed an insignificant resistance increase (10.6%) after 500 cycles of 100% stretch/release deformation. The key mechanism for the mechanical robustness is the ultrathin thickness and buckling structure. Due to the total thickness of our printed ultrathin Ag mesh transparent electrodes is less than 1.5 μm, they can be easily used in ultra-flexible photonic devices. As a demonstration, organic photovoltaics (OPVs) are fabricated using our printed Ag mesh transparent electrodes, which showed a comparable power conversion efficiency (8.3%) to those using traditional ITO electrodes (8.6%).

The insight gained from the ultra-flexible printed Ag mesh transparent electrodes and their application in ultrathin organic photovoltaics will help to apply such novel transparent electrode into other ultrathin photonic devices, and even ultra-flexible systems.

Reference:
[1] T. Yokota, et al., Sci. Adv. 2, e1501856 (2016).
[2] M. S. White, et al., Nat. Photo. 7, 811 (2013).
[3] Fukuda K, et al., Adv. Mater., 25, 29 (2017).
[4] Z. Jiang, et al., Adv. Mater. 1707526 (2018).

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