Enhanced electrical and thermal conductivity in flexible devices based on robust mechanical contact formation

Abstract

Recently, the electrical and thermal contact resistance imposed by an interface between two bodies has gained significant attention. Contact resistance is a crucial factor that directly affects the performance and power efficiency of electronic and thermal devices. Contact resistance is caused by a mismatch at the interface when two bodies come into contact. This is because the asperities of the surfaces hinder the perfect contact thus forming an air gap at the interface. A method for applying high pressure or filling the contact interface with a material of high electrical and thermal conductivity is widely used to reduce the contact resistance. In recent years, flexible and transparent electronic and thermal devices have been significantly developed owing to the development of electrical and thermal conductive nanomaterials such as carbon nanotubes (CNTs), graphene, and metal nanowires. Because these transparent and flexible devices are more sensitive than the conventional bulk devices, they are more affected by contact resistance. However, the conventional method of reducing contact resistance cannot be utilized for flexible transparent devices owing to the difficulty of uniformly pressurizing the flexible devices and the limitation of transparency. Despite the importance of contact resistance in a flexible transparent device, studies on reducing the contact resistance of flexible transparent devices are rare. Bioinspired adhesive structure, which is inspired by adhesive structures of living organisms in nature, is crucial in overcoming contact resistance in flexible transparent electronic and thermal devices. The micro- and nanoscale pillar-based bioinspired adhesive structures increase the contact area with the substrate and maximize the van der Waals (vdW) force; therefore, the device and the substrate are mechanically coupled. The strong adhesion of bioinspired adhesive structures not only attaches the flexible transparent device to the substrate, but also significantly reduces the electrical and thermal contact resistance through a strongly coupled interface. In this dissertation, we present a flexible transparent electronic and thermal device integrated with the bioinspired adhesive structures. Because of the strong adhesion of the bioinspired adhesive structure, the proposed electronic and thermal device can contact the curved surfaces without external pressure or other additives and minimize the electrical and thermal contact resistance. In Chapter 2, we introduce the self-attachable flexible transparent electronic device with low electrical contact resistance. The device has high adhesion strength, transparency, flexibility, conductivity, and low electrical contact resistance, enabling strong mechanical and electrical connections without external pressure, soldering, or vacuum deposition. Moreover, demonstrations of the flexible transparent electronic device as a smart interconnector and transparent heater were conducted. In Chapter 3, we present a flexible transparent thermal device with low thermal contact resistance. The device exhibits efficient heat transfer ability to the target substrate because of its superior adhesion strength and low thermal contact resistance with high flexibility and transparency. Compared to the conventional thermal devices, the flexible transparent thermal device reduced the thermal contact resistance by ~97% without pressing or thermal interface materials (TIMs) and showed heat transfer performance to nonplanar surfaces with uniform contact interface without air gap.