Dual effects of water on the performance of copper complex conductive inks for printed electronics

Copper complex conductive inks inevitably contains water which is derived from chemicals and humid air. The dual effects of water on the performance of copper complex conductive inks with amines as ligands were investigated for the first time. On one hand, excess water caused the deterioration of copper complexes by interacting with amines to produce high concentration of OH⁻, yielding basic salt, hydroxide or oxide precipitate, which shortened the shelf life of copper inks. The degree of deterioration was related to the amine's coordination ability and alkalinity. On the other hand, trace water could accelerate the thermal decomposition of copper complexes, promote the nucleation of copper, and yield smaller particles to form copper films with denser structure and lower resistivity. The blending of 2-ethylhexylamine (EtHex) and 2-amino-2-methyl-1-propanol (AMP) as ligands to copper formate (Cuf) could balance the dual effects of water on copper complex conductive inks. It was found that Cuf-EtHex-AMP (with 95 mol% EtHex in blended ligands) ink showed strong water resistance. The shelf life of the blended complex ink with additional 2 wt% water was still more than 2 months. The additional water enabled the formation of highly conductive copper films at low heating temperature. The resistivity of the copper film from the blended complex ink with additional 2 wt% water stored for 2 months only increased a little compared to that from the newly prepared ink (from 9.70 μΩ·cm to 14.42 μΩ·cm), which could meet the general requirements of printed electronics.