Wanli Li

Self-reducible copper inks composed of copper–amino complexes and preset submicron copper seeds for thick conductive patterns on a flexible substrate

Low-temperature and self-reducible copper inks composed of copper–amino complexes and proper submicron copper seeds are successfully developed, which can increase the copper load of inks to achieve high performance thick and dense conductive patterns. During the heat treatment, the preset copper seeds provide heterogeneous nucleation sites for metallic copper generated from the decomposition of self-reducible copper–amino complexes. These fresh copper nuclei homogeneously attach to the copper seeds to activate their surface, which contributes to the connection and neck-growth between these submicron copper seeds to achieve high conductive copper patterns. The effects of the size of copper seeds and the ratio of the amount of copper–amino complexes to seeds on the electrical resistivity and morphology of sintered copper patterns are clarified, and the functions of the heat treatment temperature and holding time are investigated. The results show that the sintered copper pattern with a high metal load of 36.8%, prepared by heat treatment at a low temperature of 140 °C for only 15 min under nitrogen atmosphere, achieves a low resistivity of 11.3 μΩ cm. Furthermore, the sintered patterns maintain a high-qualitative surface morphology and favorable thickness, and also exhibit a strong adhesion to polymer substrates which will be advantageous for the fabrication of wearable devices.

Highly reliable and highly conductive submicron Cu particle patterns fabricated by low temperature heat-welding and subsequent flash light sinter-reinforcement

Submicron Cu particle ink was developed to successfully achieve highly reliable and highly conductive Cu patterns on low-cost, transparent, and flexible substrates by an optimized two-step sintering process involving low temperature heat-welding and subsequent flash light sinter-reinforcement. The Cu ink contains a special additive of the Cu–amino complex made from copper(II) formate and 2-amino-2-methyl-1-propanol solvent. Low temperature heat-welding promotes the decomposition of the Cu–amino complex into fresh metallic Cu particles, which as nano-welders can in situ weld those big submicron Cu particles. The subsequent flash light sintering further reinforces the connection between big Cu particles with the assistance of these active nano-welders and strengthens the adhesion between sintered Cu patterns and polymer substrates due to the local soft-effect. The achieved resistivities of sintered Cu patterns on polyethylene terephthalate (PET), polyethylene naphthalate (PEN) and polyimide (PI) substrates are 26.5 μΩ cm, 15.9 μΩ cm and 7.2 μΩ cm at a low welding temperature of 140 °C for 10 min and subsequent flash light energies of 1080 mJ cm−2, 1273 mJ cm−2 and 2073 mJ cm−2, respectively, at which the same electrical properties cannot be obtained from either pure nano-Cu or submicron Cu particle ink as reported in previous research studies. Moreover, bending fatigue and oxidation-resistance tests indicate that the sintered Cu patterns have superior mechanical and environmental stability. Finally, flexible and foldable LED circuits and flexible dipole antennas were successfully fabricated to demonstrate the applicability of the sintered Cu patterns for printed electronic devices. It should be noted that this method opens a new way for making highly reliable and highly conductive Cu patterns on low-cost, transparent, and flexible substrates with big Cu particles instead of nanoparticles under a suitable sintering process, which may largely decrease the cost and enhance the application of Cu inks for flexible electronic devices.

Printable and Flexible Copper–Silver Alloy Electrodes with High Conductivity and Ultrahigh Oxidation Resistance

Printable and flexible Cu-Ag alloy electrodes with high conductivity and ultrahigh oxidation resistance have been successfully fabricated by using a newly developed Cu-Ag hybrid ink and a simple fabrication process consisting of low-temperature precuring followed by rapid photonic sintering (LTRS). A special Ag nanoparticle shell on a Cu core structure is first created in situ by low-temperature precuring. An instantaneous photonic sintering can induce rapid mutual dissolution between the Cu core and the Ag nanoparticle shell so that core-shell structures consisting of a Cu-rich phase in the core and a Ag-rich phase in the shell (Cu-Ag alloy) can be obtained on flexible substrates. The resulting Cu-Ag alloy electrode has high conductivity (3.4 μΩ·cm) and ultrahigh oxidation resistance even up to 180 °C in an air atmosphere; this approach shows huge potential and is a tempting prospect for the fabrication of highly reliable and cost-effective printed electronic devices.

Effects of oxygen containing groups on barrier layer on the stability of silver nanowire transparent electrodes

Silver nanowire (AgNW) has showed strong potential as a great alternative to indium tin oxide (ITO) film in the fabrication of transparent conductive electrode (TCE) and it especially satisfies the requirements of flexible and printable electronics. To relieve the corrosion of bare AgNWs and improve the long-term stability of AgNW-based electrodes, using barrier layer to encapsulate and overcoat of the surface of AgNWs has been proposed as one powerful solution. Reduced graphene oxide (rGO) film is used as a barrier layer extensively in the fabrication of AgNW-based electrodes. Considering the difficulty of entirely removing the oxygen containing groups (OCGs) on GO film to form rGO film, there is a lack of information about the effect of OCGs on barrier layer on the stability of AgNW-based network. Therefore, we decided to study the corrosion of AgNW network overcoated with GO film, on which contained different amounts of OCGs. Two kinds of AgNW-based electrodes, graphene oxide (GO) /AgNW/polyethylene terephthalate (PET) TCEs and rGO/AgNW/PET TCEs, were fabricated by a rapid and facile flame treatment and exposed to ambient conditions. The structural and performance changes were monitored with exposed time. Our results demonstrated that AgNWs are susceptible to the barrier layers.