Author: Wang, Shuaichen
Title: Xerogel-based Cu electrode inkjet printed on plastic substrate
Advisors: Zheng, Zijian (ITC)
Yan, Feng (AP)
Degree: Ph.D.
Year: 2022
Subject: Xerogels
Ink-jet printing
Hong Kong Polytechnic University -- Dissertations
Department: Institute of Textiles and Clothing
Pages: xxx, 174 pages : color illustrations
Language: English
Abstract: Inkjet-printed Cu electrodes can replace noble metals (e.g., Au or Ag) to fabricate high-quality flexible electronic devices. However, the inkjet-printed Cu electrodes prepared by nanoparticle ink still need a sintering process (e.g., intense light irradiation) with high energy consumption. Moreover, these kinds of sintering processes still exert instant high-temperature on the surface of the substrate, damages the flexible substrate (e.g., polyethylene (PE) and polyethylene terephthalate (PET)), the stretchable substrates, and the biological substrates. On the contrary, polymer-assisted metal deposition (PAMD) is a room-temperature metal deposition method that enables low-cost Cu electrodes to grow on plastic substrates. Nevertheless, these inkjet-printed Cu electrodes usually have a rough surface uniformity and are difficult to be used as the bottom electrodes for the fabrication of thin-film electronic devices. And the current inkjet-printing ink and appropriate treatments for the PAMD technique cannot deal with the coffee-ring effect and irregularly growing Cu.
First, we found that the mesoporous xerogel, generated from the EtOH treatment, could improve the quality of the inkjet-printed Cu electrode's smoothness, surface morphology, and graphics retention. The EtOH treatment removed the pore liquid of the gel and the copolymer cluster that did not well-bond to the overall copolymer network. The gel thin film shrunk to around half of its original thickness for converting to mesoporous xerogel after the EtOH treatment. The initial unevenness of the surface morphology of the inkjet-printed gel pattern can be reduced to half of its original after this conversion. In addition, the xerogel thin film also reduced the occurrence probability of irregular Cu that grew on the peripheral of the inkjet-printed patterns. Then, we also found 4-extra-treatments to smooth further the deposition uniformity of the inkjet-printed xerogel-based Cu electrode. This is the first report in history to show an inkjet-printed Cu electrode has a smooth surface morphology. We also found that the high-quality xerogel-based Cu electrode had a strong adhesiveness to the plastic substrate. As a proof-of-concept, we carried out a TEM cross-sectional test to demonstrate the interpenetration network of the xerogel-based Cu electrode and the Cu nanoparticles. Compared with the original gel-based Cu electrode, the xerogel-based Cu electrode had a relatively stronger polymer cluster that may help it firmly attach to the plastic substrate without the broken of the polymer cluster. The adhesive energy of the xerogel-based Cu electrode to the plastic substrate (PET) is 0.7 J/m2, which is much higher than the thermal evaporated Cr adhesively layer for thermally evaporated Cu electrode (0.06 J/m2).
Second, we used the three-solvent copolymer system as a sample to prove a new concept that a thin sheet of liquid, difficult to evaporate, can help to level the join of the inkjet-printed patterns. Besides the primary solvent, two special cosolvents were added. One cosolvent modulates the copolymer ink's viscosity, while another cosolvent, with a slow-evaporation rate, generates the thin sheet of liquid that is difficult to evaporate. As a proof of concept, we observed the drying process of the liquid sheet and the morphology of the final deposited film by an optical microscope. And the thin sheet of liquid formed a smooth joint of two inkjet-printed patterns. In addition, we found that when we inkjet-printed the patterns in a specific sequence, a well-controlled boundary will appear. Initially, only the line-shaped pattern (formed by a row of droplets with similar spacing) can create an almost perfectly border by verifying the droplet spacing. The inkjet-printed 2D pattern usually has an uncontrollable boundary due to the complex drying process of the inkjet-printed droplets. Furthermore, two of these well-controlled boundaries can form a uniform channel structure with a channel length of 15 ┬Ám without the help of any evaporation or etching techniques.
Finally, we prepared multi-layer devices, including organic thin-film transistors (OTFTs) and organic electrochemical transistors (OECTs), by only solution methods. The functional layers of the thin film devices can be directly inkjet-printed or be spin-coated on the top of the inkjet-printed Cu electrode without short current between layers benefit from the smooth surface morphology of the Cu electrode.
We also printed this kind of xerogel-based Cu electrodes onto commonly used plastic shopping bags without damaging the hate-sensitive plastic bag. This metal deposition method, powered by pure chemical energy, may replace many traditional methods in the industry of printed electronics and bring hope to human beings in the age of global warming.
Rights: All rights reserved
Access: open access

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