In order to realize a sustainable, recycling-oriented society, it is intrinsically important to develop new technologies to substitute rare elements with elements abundant in resoueces, to save energy, to introduce renewable energy, or recyce the devices.Transparent conductive oxide (TCO) films are essential materials for transparent electrodes in clean energy production sources (solar cells) and energy-saving devices (liquid crystal displays, light-emitting diodes, etc.). Indium tin oxide (ITO) is currently the most widely used TCO film. However, ITO is mainly composed of indium, a rare-earth. From the perspective of the SDGs, there is a strong need to promote the use of alternative materials to ITO, that are mainly composed of elements abundant in resources and can also be recycled.Tin dioxide (SnO2) is a TCO film in practical use during more than half a century. It is mainly composed of tin (Sn), which is abundant in resources, and has excellent weatherability and chemical resistance. It is used in solar cells, heater glass to prevent fogging, and viewing windows of analyzers. If SnO2 can replace ITO, it will have a very large impact on the realization of a recycling society.TCO films are generally patterned when used as transparent electrodes. ITO films can be easily patterned by wet processes using commercially available etchants the photoresist masks fabricated by photolithography. In contrast, SnO2 films are quite difficult to pattern by wet processes due to its high chemical resistance. SnO2 films are mainly patterned by dry processes, such as laser processing or sandblasting. However the former is expensive in cost, and the latter may cause substrates to break after shipping. If SnO2-based TCO films can be patterning by wet process, it will directly lead to improved throughput, cost reduction, and lower product defect rate, and will be a step forward toward the major goal of replacing ITO.We developed a new patterning method of SnO2-based TCO films by two-step wet processes. In the first step, the SnO2 was reduced to metallic Sn by applying voltage in gluconic acid aqueous solution (Figure 1). The reduction to Sn was confirmed by X-ray diffraction measurements. Cross-sectional observation by a scanning electron microscope (SEM) revealed that reduction proceeds from the surface of the SnO2 film toward the inside.In the second step, Sn was removed by acid, base, or electrolysis. The complete removal of the SnO2 film was confirmed by SEM and energy-dispersive X-ray spectroscopy. In addition, transmittance and reflectance of the glass substrate after removal of SnO2 film were measured.It was found that patterning SnO2-based TCO films in the micrometer order scale is possible by combining the developed two-step method with the photoresist masks fabricated by photolithography. Figure 1
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