Hybrid Printing Metal-mesh Transparent Conductive Films with Lower Energy Photonically Sintered Copper/tin Ink
Abstract
With the help of photonic sintering using intensive pulse light (IPL), copper has started to replace silver as a printable conductive material for printing electrodes in electronic circuits. However, to sinter copper ink, high energy IPL has to be used, which often causes electrode destruction, due to unreleased stress concentration and massive heat generated. In this study, a Cu/Sn hybrid ink has been developed by mixing Cu and Sn particles. The hybrid ink requires lower sintering energy than normal copper ink and has been successfully employed in a hybrid printing process to make metal-mesh transparent conductive films (TCFs). The sintering energy of Cu/Sn hybrid films with the mass ratio of 2:1 and 1:1 (Cu:Sn) were decreased by 21% compared to sintering pure Cu film, which is attributed to the lower melting point of Sn for hybrid ink. Detailed study showed that the Sn particles were effectively fused among Cu particles and formed conducting path between them. The hybrid printed Cu/Sn metal-mesh TCF with line width of 3.5 μm, high transmittance of 84% and low sheet resistance of 14 Ω/□ have been achieved with less defects and better quality than printed pure copper metal-mesh TCFs.
Introduction
Printed electronics is a new emerging filed in recent years1,2. Compared with conventional vacuum deposition and photolithographic techniques, printing is low-cost, efficient for material usage3,4, has been applied to fabricated flexible displays, radio frequency identification tags (RFID)5,6, and wearable electronics7,8.
Conductive ink based on silver nanoparticle is widely used in printed electronics9,10, albeit at high cost11. There have been continuous efforts in developing copper ink, either based on copper or copper oxide nanoparticles, because of the low price of copper. However, copper nanoparticles are too easily to oxidize and cannot be sintered by traditional sintering methods under ambient condition12,13. The photonic sintering using intensive pulse light (IPL) has proved to be effective for copper nanoparticle ink14,15. However, the high photonic sintering energy, which is needed to make copper nanoparticle films conductive, leads to the destruction of samples16.
One of the important applications of conductive inks is to print conductive electrodes. There has been a considerable surge of activity in the development of printing metal grids as an alternative to ITO for transparent conductive films (TCFs)17,18,19,20. Among the many reported techniques, the authors’ group developed a hybrid printing process which prints conductive ink into a narrow trench instead of conventionally on surface (The hybrid printing process is shown in Supplementary Fig. S1). In this innovative process, less than 3 μm grid width can be achieved and the embedded Ag metal-mesh TCF has extremely low sheet resistance (<0.5 Ω/□) while still maintains high transparency (>85%)21. The hybrid printed TCFs have been successfully used in touch-screen panels and solar cells21,22,23. The authors’ group also conducted research on copper ink and used an IPL for photonic sintering24. Unlike Ag ink, Cu ink usually needs very high energy photonic sintering to convert copper oxide into copper. Though high energy photonic sintering poses no problem for copper ink printed on a flat surface, it is problematic if the copper ink is embedded into a trench. Defects in the forms of broken mesh lines and missing copper materials in the trenches happened in the process.
In this study, a Cu/Sn hybrid ink has been developed by mixing Cu and Sn particles. The sintered energy of Cu/Sn film with mass ratio of 2:1 (Cu:Sn) was found 21% less than that for Cu film, which is attributed to the lower melting point of Sn for hybrid ink. The SEM inspection revealed that the molten Sn particles were fused among Cu particles and formed conducting paths on Cu/Sn hybrid film. Using the Cu/Sn ink and previously developed Cu ink at the authors’ group for comparison, metal-mesh TCFs were fabricated by the hybrid printing process and sintered with an IPL. The Cu/Sn metal-mesh TCF achieved the transmittance of 84% and low sheet resistance of 14 Ω/□ with less defects and better quality than printed pure copper metal-mesh TCFs.
Results
Photonic sintering of screen-printed Cu and Cu/Sn films
In order to study the sintering characteristics of Cu/Sn hybrid film and to compare with the sintering of pure Cu film, the Cu and Cu/Sn films with the size of 2 cm * 2 cm (as shown in the inset of Fig. 1) were fabricated by screen-printing and the IPL energy density ranging from 2.64 to 5.02 J/cm2 was applied. Figure 1 presented the sheet resistance of sintered Cu and Cu/Sn hybrid film with respect to irradiation energy. Cu/Sn films of mass ratio 2:1 and 1:1 (Cu:Sn) sintered by 2.68 J/cm2 exhibited weak conductivity. And sheet resistance was rapidly down with the increase of sintered energy density. However, the Cu film was non-conductive until sintered by 3.25 J/cm2. I (...truncated)