Understanding the Importance of Cu(I) Intermediates in Self-Reducing Molecular Inks for Flexible Electronics

Fast and scalable low-temperature deposition of microscale metallic features is of utmost importance for the development of future flexible smart applications including sensors, wireless communication, and wearables. Recently, a new class of metal-organic decomposition (MOD) copper inks was developed, consisting of self-reducing copper formate containing amine complexes. From these novel inks, copper metal features with outstanding electrical conductivity (±10⁵ S cm^-1) are deposited at a temperature of 150 °C or less, which is well below the reduction temperature of orthorhombic α-copper formate (around 225 °C). However, the underlying principle of this reaction mechanism and the relationship between the corresponding temperature shift and the amine coordination are still under debate. The current study provides a full explanation for the shift in reduction temperatures via in situ characterization. The results clearly indicate that the structural resemblance and stability of the Cu(II) starting compound and the occurring Cu(I) intermediate during the in situ reduction are the two main variables that rationalize the temperature shift. As such, the thermal compatibility of copper MOD inks with conventional plastic substrates such as polyethylene terephthalate can be explained, based on metal-organic complex properties.