Fabricating Air-Stable Copper Films with Exceptional Electrical Conductivity without Sintering

Copper is an excellent electrical conductor, nearly matching the conductivity of silver (Cu: 5.96 × 10⁷ S/m <i>vs </i>Ag: 6.30 × 10⁷ S/m), but at a fraction of the cost (Cu: $ 0.28/oz <i>vs </i>Ag: $ 21.6/oz) making it an ideal choice for cost sensitive large area applications which are in high demand. However, unlike silver, copper has a much higher susceptibility towards oxidation, a risk that is particularly acute in particulates and suspensions with high surface to volume ratio, which generates additional processing complications including the need for surface passivation and inert environments for processing as well as sintering. The use of fossil fuel based long-chain hydrocarbon ligands to passivate copper is the common approach in the field but decomposition of such ligands to fabricate functional conductive films require high sintering temperatures and controlled conditions. Moreover, with the rapid growth of the flexible and wearable electronics industry, sintering becomes increasingly less practical as most substrates used in this sector are temperature sensitive. Herein, a new approach to fabricating conductive air-stable copper films with resistivities close to that of bulk copper is demonstrated which only requires compression at room temperature, completely removing the need for any form of sintering. This approach facilitates the generation of highly conductive simple unsupported copper films as well as intricate patterns on paper and printed circuit board substrates with an aqueous, biopolymer stabilised copper ink formulation. The remarkable conductivity and oxidative stability of the copper films, coupled with the sustainability of the approach have the potential to precipitate a paradigm-shift in the use of copper inks for printable electronics.