Enhancing adhesion properties between binder-free copper nanoink and flexible substrate using chemically generated interlocking structure

Abstract Organic residues formed when using organic binders to increase the adhesive strength of Cu ink to PI substrates degrade the electrical conductivity of Cu electrodes. Binder-free Cu ink can be used to fabricate highly conductive Cu electrodes but lead to poor adhesion owing to inherently weak adhesion between the polymer substrates and metals. We therefore propose a facile method to enhance adhesion between a printed copper electrode formed by flash light sintering and a polyimide (PI) substrate without sacrificing the electrical conductivity by forming an interlocking structure at the interface between the Cu electrode made of binder-free ink and PI substrate through simple chemical treatment. The surface of the PI substrate was converted to polyamic acid (PAA) via potassium polyamate formation after sequential chemical treatment using potassium hydroxide and hydrochloric acid. During the flash light sintering process, the PAA layer, which is much softer than PI, could easily migrate into the cavities between the sintered Cu nanoparticles due to the pressure difference originating from evaporation of H 2 O released through reduction of the oxide shell of Cu nanoparticles. Thus, an interlocking structure was formed and the substrate surface quickly reverted to PI by absorbing thermal energy during sintering. The increased contact area between Cu nanoparticles and the substrate surface of the interlocking structure could explain the dramatic improvement of the adhesion strength of the Cu electrode from 0 B to 5 B, which corresponds to the highest class of adhesion strength. In addition, the resistivity of the Cu electrode fabricated in this process was measured to be 2.5 times larger (4.17 μ Ω·cm) than that of bulk Cu, which has not been achieved for printed Cu electrodes before. Our results could lead to new perspectives in the field of printed electronics.