Nickel oxide-based flexible thin-film NTC thermistors by using reverse offset printing

In recent years, the use of printing methods to fabricate electronic devices (printed electronics) has attracted attention because of their low cost and low environmental impact. Printing technology enables the high-throughput fabrication of electrical circuits on film substrates, thereby providing inexpensive personal healthcare devices to monitor health status in real-time, for example. Temperature detection is one of the central concerns as a fundamental physical quantity in various fields. In 2013, a highly sensitive flexible thermistor was reported by formulating aqueous inks of nickel oxide (NiO) nanoparticles for inkjet printing. However, it required a high-temperature calcination process of more than 200 °C, which led to the use of expensive polyimide films with high heat resistance. It is necessary to promote further the development of low-temperature processes for printed thermistors to realize flexible negative temperature coefficient (NTC) thermistors at low cost using printed electronics technology. Moreover, in screen printing and inkjet printing, the definition of the ink pattern applied on the substrate changes due to spreading and coffee distortion phenomena, and the thickness between sensors becomes non-uniform, which is a structural consistency problem that can lead to variations in sensing performance. Therefore, in this study we developed low-temperature processable printed NTC thermistors with a temperature-sensitive layer of NiO by using reverse offset printing. The NTC thermistors were fabricated by printing a comb-like pattern of silver nanoparticles and a thin NiO film. In addition, the low-temperature formation of a NiO layer by oxygen plasma treatment was investigated, and x-ray photoelectron spectroscopy was used to carry out compositional analysis of the surface. Together with the plasma-assisted calcination, a flexible NTC thermistor formed on polyethylene terephthalate film is demonstrated.