Contribution of Ligand Oxidation Products to High Durability of Copper Films Prepared from Low‐Sintering‐Temperature Copper Ink on Polymer Substrates

Abstract

In electronic printing, ensuring high durability of sintered copper films on flexible substrates to obtain environmental stability and mechanical flexibility has become the most important task from a practical standpoint. In the study reported here, the authors develop solution synthesis of 2‐amino‐2‐ethyl‐1,3‐propanediol (AEP)‐protected copper nanoparticles (AEP–Cu NPs) with sizes of 3–8 nm in ethylene glycol, where the Cu NPs are stabilized via the metallacyclic coordination stability of the AEP ligands. The sintered Cu film exhibits a resistivity of 50 μΩ cm−1 after heating at 150 °C under a nitrogen atmosphere. Most importantly, the resulting Cu films on the polyethylene terephthalate (PET) substrates show excellent durability in terms of bending and adhesion without requiring any additives like nanotubes and nanowires. Furthermore, the authors successfully demonstrated the high environmental stability of the resulting Cu film even after it is exposed to harsh environmental conditions (RH 80%, 60 °C) for 1 month. The environmental durability is further improved by utilizing a composite ink of AEP–Cu NPs with copper microflakes. It is experimentally proven that oxidation products from AEP ligands originating in the sintering process contributed to the high durability of sintered copper films on flexible substrates.

Formulation of Screen-Printable Cu Molecular Ink for Conductive/Flexible/Solderable Cu Traces

“…While commercial Novacentrix CuO NP ink inkjet printed on silica-coated PET (Novele) can produce ∼10 μΩ•cm, 56,57 there is currently no screen-printable formulation with adequate mechanical properties and air stability of traces on conventional PET. Further comparisons include Kawasaki et al, 15 where PET coated with Cu composite nanoink consisting of submicron/nano-Cu particles with a roller bar resulted in volume resistivity of 50 μΩ•cm when sintered at 150 °C under N 2 . However, the screen printability of composite nanoinks was not evaluated.…”

“…Recently, several studies have been carried out to obtain conductive Cu films on flexible substrates particularly focusing on low-temperature sintering of Cu nanoparticle (CuNP) inks, − different precursor metal-organic decomposition (MOD) inks, − and composite inks [MOD and particles/flakes/carbon nanotube (CNT)]. − MOD inks represent an attractive option as they are particle free and can be formulated for a wide range of printing techniques without flocculation or sedimentation. To increase the Cu loading and/or decrease the amount of excipient organics in the MOD ink, CuNP or submicron-size particles may be added.…”

Filtration-induced production of conductive/robust Cu films on cellulose paper by low-temperature sintering in air

“…Bending tests on the sintered Cu films were conducted under repeated tensile and compressive loading on a custom-made bending tester [44]. The bending tester consisted of a clasp to hold the Cu films in position on the substrates and a moving part to bend the Cu films using an electronic motor.…”

Design and applications of stretchable and self-healable conductors for soft electronics

“…In order to fully realize the advantages of copper (e.g., comparable electrical conductivity with silver), a diversity of strategies to avoid the oxidation problem without compromising electrical conductivity have been devised. They include coating with graphene oxide, nickel or chitosan [105, 106], or ligand [107], reduction of the oxide layer to metallic copper by acid solution [108], or embedment into elastomer [71, 82].…”