A novel simple laser digital patterning process to fabricate Ni-based flexible transparent conducting panels using solution-processed nonstoichiometric nickel oxide (NiO x ) thin films and their applications for flexible transparent devices are reported in this study. A largescale synthesis route to produce NiO x nanoparticle (NP) ink is also demonstrated. A low-power continuous-wave laser irradiation photothermochemically reduces and sinters selected areas of a NiO x NP thin film to produce Ni electrode patterns. Owing to the innovative NiO x NP ink and substantially lowered applied laser power density, Ni conductors can be fabricated, for the first time to the best of the authors' knowledge, even on a polyethylene terephthalate substrate, which is known to have one of the lowest glass-transition temperatures among polymers. The resultant Ni electrodes exhibit a high-temperature oxidation resistance up to approximately 400 °C, and high corrosion resistance in tap water and even in seawater. Moreover, a superior mechanical stability of the Ni conductors is confirmed by tape-pull, ultrasonic-bath, bending/twisting, and cyclic bending (up to 10 000 cycles) tests. Finally, flexible transparent touch screen panels and electrical heaters are fabricated with mesh-type Ni conductors to demonstrate possible applications.market. However, ITO suffers from several limitations such as brittleness and lack of flexibility of ITO layers, and costly deposition method requiring vacuum-based equipment, which hinders its application to the next-generation electronics, particularly to flexible or stretchable electronics. Therefore, the demand for alternative materials and deposition methods of TCEs is rapidly increasing. Among alternatives to conventional deposition or patterning methods for conducting/semiconducting electrodes, a laser digital patterning process on solution-processed nanoparticle (NP) thin films [2] has recently attracted significant attention since it enables to achieve a photolithography-free, low-cost, and ondemand electrode fabrication. The laser digital patterning process, in particular, can be applied to fabricate flexible electronic devices owing to lowered thermal stress exerting on thermally vulnerable polymer substrates (see expanded discussion in Section S1, Supporting Information). As materials for laser digital patterning in the early stage, NPs or nanocomposites composed of noble metals such as Ag or Au have been widely employed [3] because these nanomaterials are oxidation resistant, and thus can be easily formed to conducting electrodes simply by sintering. However, nowadays, there is an increasing demand to diversify the electrode materials not only owing to the high price of the noble metals but also particularly owing to the need for unique properties of relevant Flexible Conducting PanelsThe ORCID identification number(s) for the author(s) of this article can be found under https://doi.

As an alternative approach to the conventional deposition and photolithographic processes, the laser digital patterning (LDP) process, which is also known as the laser direct writing process, has attracted considerable attention because it is a non-photolithographic, non-vacuum, on-demand, and cost-effective electrode fabrication route that can be applied to various substrates, including heat-sensitive flexible substrates. The LDP process was initially developed using noble metal nanoparticles (NPs) such as Au and Ag because such materials are free from oxidation even in a nanosize configuration. Thus, the NPs must be fused together to form continuous conductive structures upon laser irradiation. However, common metals are easily oxidized at the nanoscale and exist in oxidized forms owing to the extremely large surface-to-volume ratio of NPs. Therefore, to fabricate conductive electrodes using common metal NPs via the LDP process, laser irradiation should be used to sinter the NPs and simultaneously induce additional photochemical reactions, such as reduction, and defect structure modification to increase the conductivity of the electrodes. This review summarizes recent studies on the LDP process in which metal oxide NPs, such as ITO, ZnO, CuO, and NiO, were exclusively utilized for fabricating conductive electrodes. The outlook of the LDP process for these materials is also discussed as a method that can be used together with or as a replacement for conventional ones to produce next-generation transparent conductors, sensors, and electronics.

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