Solution-based β-diketonate silver ink for direct printing of highly conductive features on a flexible substrate

Abstract

The novelty of this study is the laboratory formulation of silver ink adapted for the inkjet printing of conductive metallic features on flexible polyimide (PI) substrates with potential integrated circuit applications ranging from large-area electronics to low-end applications. A new silver precursor for printing conductive patterns with the empirical formula [Ag(dien)](tmhd), where tmhd = 2,2,6,6-tetramethyl-3,5-heptanedionato and dien = diethylenetriamine, was synthesised using a simple and environmentally friendly method. The viscosity and surface tension of the organic solvent system were optimised through the addition of ethyl cellulose and hexylamine, yielding potential printing ink of high Ag wt%. Silver patterns on a flexible PI substrate were produced by thermal annealing of silver features prepared either by spin-coating or by directly drawing with a piezoelectric inkjet printer. Films were produced using a silver precursor (60 wt%) dissolved in hexylamine (39 wt%) and ethyl cellulose (1 wt%) with a viscosity of 9–11 mPa and annealed in air at 250 °C. They displayed resistivity values in the range of 4.625–9.376 × 10−6 Ω cm. The composition of printing ink is [Ag(dien)](tmhd) : hexylamine : ethyl cellulose = 45 : 54 : 1 by wt%. A resistivity of 7.44 × 10−6 Ω cm was found for a silver line with a width of 177 μm and a thickness of 106 nm. The silver patterns were characterised by scanning electron microscopy, FT-IR, X-ray photoelectron spectroscopy and X-ray diffraction. The resistivities of our silver patterns are lower than those previously prepared by other research groups using water-based silver salts as ink. We propose that the high Ag wt% achievable with an organic solvent-based system may explain this lower resistivity.