5. Substrate and Encapsulation Materials for Printed Flexible Electronics

Substrate, dielectric, and encapsulation materials are critically important as their properties can dominate those of the integrated flexible electronic system. A flexible substrate should be highly deformable and mechanically robust and must exhibit high tolerance levels of bending repeatability. They are also required to possess properties such as dimensional stability, thermal stability, low coefficient of thermal expansion (CTE), excellent solvent resistance, and good barrier properties for moisture and gases. The substrate materials mainly include plastic films, metal foils, and fibrous materials (including paper and textiles). Moreover, a uniform layer of dielectric is needed to promote the activation of the medium caused by electric fields or other transduction phenomena. Inorganic materials such as silica, alumina, and other high permittivity oxides often used in electronics on flexible substrates are usually not printable. Low-cost organic dielectric materials that are available in large quantities and can be dissolved in various solvents and solutions can be printed easily as compared to inorganic counterparts. Some of the commonly used organic dielectric materials in printed electronics are poly (4-vinylphenol) (PVP), poly(methyl methacrylate), Polyethylene Terephthalate, Polyimide, Polyvinyl alcohol, and Polystyrene. Besides dielectric layers in electronic devices, solution processed organic dielectric materials are also used for final encapsulation of printed devices. There is a wide range of permeation requirements for different encapsulation materials. To ensure protection of flexible devices, conventional encapsulation methods are not suitable due to their inherent rigidity, and organic/inorganic hybrid thin-film encapsulation (TFE) has been considered as the most promising technology. This chapter will provide a brief review on substrate, dielectric, and encapsulation materials for flexible electronics applications.