Abstract

Silver (Ag), gold (Au), and copper (Cu) have been utilized as metals for fabricating metal-based inks/pastes for printed/flexible electronics. Among them, Cu is the most promising candidate for metal-based inks/pastes. Cu has high intrinsic electrical/thermal conductivity, which is more cost-effective and abundant, as compared to Ag. Moreover, the migration tendency of Cu is less than that of Ag. Thus, recently, Cu-based inks/pastes have gained increasing attention as conductive inks/pastes for printed/flexible electronics. However, the disadvantages of Cu-based inks/pastes are their instability against oxidation under an ambient condition and tendency to form insulating layers of Cu oxide, such as cuprous oxide (Cu2O) and cupric oxide (CuO). The formation of the Cu oxidation causes a low conductivity in sintered Cu films and interferes with the sintering of Cu particles. In this review, we summarize the surface and interface designs for Cu-based conductive inks/pastes, in which the strategies for the oxidation resistance of Cu and low-temperature sintering are applied to produce highly conductive Cu patterns/electrodes on flexible substrates. First, we classify the Cu-based inks/pastes and briefly describe the surface oxidation behaviors of Cu. Next, we describe various surface control approaches for Cu-based inks/pastes to achieve both the oxidation resistance and low-temperature sintering to produce highly conductive Cu patterns/electrodes on flexible substrates. These surface control approaches include surface designs by polymers, small ligands, core-shell structures, and surface activation. Recently developed Cu-based mixed inks/pastes are also described, and the synergy effect in the mixed inks/pastes offers improved performances compared with the single use of each component. Finally, we offer our perspectives on Cu-based inks/pastes for future efforts.

Highlights

  • Copper (Cu) is one of the significant metals in the industry that can be used in various applications

  • Multiple washing operations require as-prepared colloidal Cu@Ni NPs (Cu) nanoparticles to remove the excess protective agents, the unreacted reagents, and the adsorbed inorganic salts generated in the reaction, as they would often increase the electrical resistivity of the final conductive inks

  • Various factors on the Intense pulsed light (IPL) sintering need to be multiple heating/drying process) on the IPL sintering need to be established by optimizing individual Cu-based inks/pastes

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Summary

Introduction

Copper (Cu) is one of the significant metals in the industry that can be used in various applications. Ag nanoparticle-based inks can produce conductive films at low-temperature sintering (less than 150 ◦C), and this is the advantage of the formation of conductive patterns on flexible substrates with a low heat resistance. These surface control approaches include surface designs by polymers, small ligands, core-shell structures, and surface activation.

Surface Oxidation Behavior of Cu Nanoparticles
Surface Designs by Polymers
Gelatin
Surface Designs by Small Organic Ligands
Oleylamine
Alkanolamine
Short-Chain Carboxylic Acids
Oleic Acid
Surface Designs by Core-Shell or Alloy Structure of Nanoparticle
Copper Oxide Shell
Surface Activation of Micron-Sized Cu Particles for Low-Temperature Sintering
Adhesion Enhancement of Sintered Cu Films on a Flexible Substrate
Findings
Conclusions and Outlook

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