Absorber-Free Laser Welding of Transparent Thermoplastic Films via Expanded Beam for Flexible Displays

For the commercialization of next-generation displays for flexible and rollable devices, transparent joints with flexibility and mechanical robustness are essential. Recent trends demand novel bonding methods not only for device packaging but also for enabling advanced structural designs that overcome the limitations of conventional techniques. In this study, we present an absorber-free laser bonding method for flexible substrates, achieved using an expanded laser beam and applied pressure to form damage-free, transparent joints. Precision surface welding was induced at the substrate interface through localized heating, while keeping both the total thickness variation and the interfacial deformation at the submicron level. Under controlled laser power and irradiation time, the morphology and transparency of the bonded joint were characterized. The bonding mechanism was investigated based on cross-sectional inspections, including scanning electron microscopy and transmission electron microscopy. Finally, the practical applicability of the proposed method was demonstrated through the fabrication of a prototype wearable watch.