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

Abstract

The fast and scalable low-temperature deposition of nanoscale metallic features is of the utmost importance for the development of future flexible smart applications including sensors, wireless communication and wearables. Recently, a new class of metalorganic 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 (± 10 μΩ cm) are deposited at temperatures 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 is 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 the copper MOD inks with conventional plastic substrates like polyethylene terephthalate (PET) can be explained, based on the metalorganic complex properties.

Scalable Approaches to Copper Nanocrystal Synthesis under Ambient Conditions for Printed Electronics

“…Moreover, OlNH 2 is known to reduce the overall decomposition temperature of the complex, originally around 200 °C for pure , to only 120 °C. , Different hypotheses have been presented to explain this decrease in reduction temperature. Marchal et al argued, for example, that the in situ reduction mechanism of copper formate proceeds via a transient Cu­(I) intermediate . Compared to other Cu­(II) complexes, it was inferred that the decrease on the reduction temperature arose from the low stability of the Cu­(I) intermediate and the structural differences between Cu­(II) and Cu­(I) species along the thermal decomposition route.…”

“…Marchal et al argued, for example, that the in situ reduction mechanism of copper formate proceeds via a transient Cu(I) intermediate. 45 Compared to other Cu(II) complexes, it was inferred that the decrease on the Cu(HCO ) 2 2 reduction temperature arose from the low stability of the Cu(I) intermediate and the structural differences between Cu(II) and Cu(I) species along the thermal decomposition route. Such a two step decomposition has indeed been confirmed for the synthesis of Cu NCs from Cu(HCO ) 2 2 used here, 40 where the reduction from Cu(II) to Cu(I) makes the originally blue reaction mixture turn pale at 120 °C.…”

Layer Morphology and Ink Compatibility of Silver Nanoparticle Inkjet Inks for Near-Infrared Sintering

“…Inkjet printing can be performed with different types of inks. For printed electronics, the most applied ink types are metal nanoparticle inks [10], metal-organic decomposition inks (MOD) [11,12], polymer inks and (graphene) micro flake inks [13,14]. In this study, only metal nanoparticles are studied.…”

Effectiveness of Ligand Denticity-Dependent Oxidation Protection in Copper MOD Inks

“…Although amine coordination significantly lowers the required in situ reduction temperature (down to 100 °C), 16 adequate oxidation protection during the thermal treatment still remains an important concern. This is mainly attributed to the convoluted chemistry involved, including the transition via a Cu(I) intermediate, as first reported by Farraj et al 10 Furthermore, the in situ reduction temperature and the reaction mechanism can be related to both the Cu(II) and Cu(I) complex geometries 17 as the type of amine involved in the coordination. 18 For this reason, almost all studies report on the decomposition of copper MOD inks in a nitrogen 19−25 or even reducing atmosphere (H 2 /N 2 or formic acid).…”

“…The protective mechanism was interpreted in the recently reported framework of Cu(I) intermediates influencing the copper MOD reduction pathway. 17 In concordance with the ligand nature and the resulting copper complex structure, the dependency of oxide formation on multiple experimental parameters such as the curing time and air flow was confirmed.…”