Metal particle-free inks for printed flexible electronics
Metal particle-free inks, composed of metal salts or metal complexes and volatile solvents, have received significant attention due to their flexibility in preparation, excellent stability and a relatively low sintering temperature in comparison with metal nanoparticle-based inks (nano inks). Formulating such inks has the potential to solve the problems occurring in the synthesis and patterning processes of metal nano inks by careful design of the ink formulation. In this paper, the development of silver and copper particle-free inks is reviewed, with particular attention on the ink formulation, patterning and post-treatment methods. The challenges are also discussed. The properties of the silver particle-free ink vary primarily with the type of silver precursor and the ligand used to solubilize and stabilize it. Highly conductive silver films with controlled microstructure features can be obtained at low sintering temperatures by the selection of appropriate precursor materials and ligands. Printability is also an issue to be solved to obtain patterns with high resolution for practical applications of these inks, which can be realized by adjusting the fluidic properties of the ink and the printing parameters. In terms of copper particle-free inks, the precursors available for selection are very limited and most of them are based on copper(II) formate. New precursors should be exploited. Besides, new sintering methods can also be explored for energy-efficient manufacturing of flexible electronics. In this respect, self-sintering, where sintering occurs spontaneously in the patterning process, is preferred. A Cu–Ag hybrid particle-free ink is also a possible choice for low-temperature fabrication of conductive patterns due to the catalytic property of silver. Finally, it will be advantageous to exploit the possible applications of such inks in the area of electronic and energy devices.