Adhesive-free joining and application for flexible devices - a review

Abstract

Interface has been recognized for a long time as a part of materials such as surface and grain boundary in bulk in spite of the fact that interface gives decisive influences on the performance and durability of devices in practical use. Interface is also well known to have four principal performances and roles such as mechanical, electrical, magnetic and optical properties. Here, I will at first review the interface behavior with special emphasis on materials science and then introduce typical examples of case study. Mechanical behavior of interface can be attributed to joining of different materials in particularly at low temperature and intrinsic defects introduced in the process of joining caused by difference in thermal expansion. Defects thus introduced is known to result in work hardening of interface. Recently, an epoch-making method of joining to utilize water gas adsorption layer as an adhesive agent. This gas adsorption joining (GAJ) was demonstrated in cycloolefin polymer (COP) film and borosilicate glass system. Both surfaces were first modified by atmospheric pressure plasma, followed by close lamination to reaction distance and heated at about 100°C to result in the formation of a strong covalent bond for joining. Durability thus formed system was found to remain unchanged even after weathering test in 60°C, 95%RH for 2000 hrs. Joining mechanism can be explained in terms of an increase in density of functional molecules at the interface, followed by dehydration. Thickness of joining layer was confirmed by X-ray photoelectron spectroscopy and STEM and EELS to be 1∼2nm. Sealing performance of GAJ was examined by measuring Water Vapor Transmission Rate (WVTR) by MOCON system using joined PET films on which high barrier layer was coated. The WVTR of line sealing with 1 mm width and 1-2 nm thick was found to be better than 5 x 10−4 g/m2/day. Practical applications of thus developed GAJ for joining and sealing are now in progress for flexible film devices such as OLED display, solar cells, and healthcare devices. Finally, roles of interface in materials science in future will be critically discussed.