Device Packaging 2019

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Smart Passivation Materials with a Liquid Metal Microcapsule as Self-Healing Conductors for Sustainable Electronic Devices
Keywords: self healing, liquid metal, passivation
Currently, there is a high demand for flexible displays but planar and rigid wafer-based electronic devices are intrinsically incompatible with deformable organic systems. Thus, organic semiconductors and conducting polymers have been pursued for large-area electronics because of their intrinsic flexibility, light weight, and low costs. The electrical failure of conductive pathways in highly integrated circuits results in the loss of function, hindering advanced electronic packaging. Therefore, a passivation layer has been essential to preserve the soft circuits and provide self-healing of the electrical pathways after they are damaged. In this study, we synthesized liquid metal microcapsules (LMCs), i.e., liquid metal colloids encapsulated with urea-formaldehyde (UF), by employing in-situ polymerization. Strong ultrasonication of the liquid metal under a high stirring speed created small colloids with an average diameter of 5.5 μm. This protocol was very simple but it facilitated mass production of uniform and small LMCs. Further, passivation films were easily prepared by mixing the LMCs with a liquid prepolymer [epoxy and poly(urethane acrylate) (PUA)]. They were highly miscible at the early stage. However, phase separation occurred between the capsules and the prepolymer due to differences in their densities. After curing the composite, free-standing films were made that showed excellent healing efficiency of damaged metal conductors. The self-healing mechanism revealed that the outflow of liquid metal from ruptured capsules immediately filled the damaged sites. We also investigated the mechanical properties of LMCs with different diameters. High-quality LMCs were used as the passivation layer of a metal contact in flexible perovskite solar cells. Because thermal annealing of epoxy can damage perovskite, ultraviolet (UV)-curable PUA was employed as the polymeric matrix of the LMCs. The LMC/PUA composite layer immediately healed the damaged Au contact of the devices, leading to outstanding recovery and retention of the power conversion efficiency (PCE) for both the backward and forward sweep (initial PCE = 15.07%, and recovered PCE = 14.94% for BW sweep). We believe that the surface passivation and electrical healing of metal interconnect realized from the polymer/LMC composite would provide the high sustainability of electronic devices.
Kunmo Chu, Principal Researcher
Samsung Advanced Institute of Technology
Suwon, Gyeonggi
South Korea

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