Insight into the Types of Alkanolamines on the Properties of Copper(II) Formate-Based Conductive Ink

Conductive inks are one of the most important functional materials for printed flexible electronic devices, and their properties determine the methods of subsequent patterning and metallization. In comparison with copper nanoparticle or nanowire inks, copper particle-free inks employing copper(II) formate (Cuf) as a precursor have attracted the interest of researchers due to their flexibility in preparation, excellent stability, and lower conversion temperature. Alkanolamines can provide Cuf with excellent solubility in alcohols while being less toxic and having a certain reducibility, making them preferable ligands in comparison with aliphatic amines and pyridine. However, there have been few studies on the effects of the alkanolamine types on the performance of Cuf inks. Also, the decomposition mechanism of copper-alkanolamine complex inks is not clear. In this work, different kinds of alkanolamines were chosen as ligands to formulate Cuf inks to address the mentioned issues. The influences of amine types on the stability, wettability, thermal decomposition behavior, and electrical performance of the formulated Cuf particle-free inks were investigated in detail. The results show that the utilization of alkanolamines could provide Cuf with excellent solubility in alcohols, resulting in an ink with good stability and favorable wetting properties. The thermal decomposition temperature and electrical performance of the formulated copper ink are largely dependent on the amine used. When amines with a longer carbon chain and more branches were utilized to prepare the ink, a decreased decomposition temperature was observed on the derived inks because of the steric hindrance effect. Copper films with good morphology and conductivity could be obtained at low temperatures by selecting the appropriate alkanolamine. Copper particle-free conductive ink from 2-amino-2-methyl-1-propanol demonstrated better morphology and electrical performance (16.09 μΩ·cm) and was successfully used for conductive circuits by direct-writing.

The effects of complex agent and sintering temperature on conductive copper complex paste

In this research study, the formulation of precursor-type Cu pastes was done by mixing copper (II) acetate and 2-amino-2-methyl-1-propanol (AMP). Accordingly, the influence of the complex agent amount on the Cu pastes stability was examined at diverse mole ratio ratios. The Cu paste's optimal formulation was then obtained with the copper acetate to AMP ratio of 0.5. The Cu paste with a CuA/AMP ratio at 0.5 was then coated onto glass substrates; they were then sintered under an N₂ environment at various temperatures including 140, 180 and 220 °C. Characterization of the copper complex paste was done by Fourier transform infrared (FTIR) spectroscopy, UV-vis spectroscopy as well as Thermal analysis. Pattern's characterization was also done through X-ray diffractometry (XRD), field emission scanning electron microscope (FE-SEM) as well as four-point probe method for the purpose of confirming the related crystal structure, microstructure and electrical conductivity, respectively. It is demonstrated how the sintering temperature could affect the Cu film. The printed patterns on the glass substrate which was cured at the temperature of 180 °C displayed metalized cooper with low resistivity (30 μΩ cm) and dense copper films.

Molecular Copper Decomposition ink for Printable Electronics

Nanostructured metal materials are the frontrunners of numerous electronic advancements. While realizing such potential, it is indispensable to address its oxidation and stability drawbacks, that are due to its high...

MODs vs. NPs: Vying for the Future of Printed Electronics

This Minireview compares two distinct ink types, namely metal-organic decomposition (MOD) and nanoparticle (NP) formulations, for use in the printing of some of the most conductive elements: silver, copper and aluminium. Printing of highly conductive features has found purpose across a broad array of electronics and as processing times and temperatures reduce, the avenues of application expand to low-cost flexible substrates, materials for wearable devices and beyond. Printing techniques such as screen, aerosol jet and inkjet printing are scalable, solution-based processes that historically have employed NP formulations to achieve low resistivity coatings printed at high resolution. Since the turn of the century, the rise in MOD inks has vastly extended the range of potentially applicable compounds that can be printed, whilst simultaneously addressing shelf life and sintering issues. A brief introduction to the field and requirements of an ink will be presented followed by a detailed discussion of a wide array of synthetic routes to both MOD and NP inks. Unindustrialized materials will be discussed, with the challenges and outlook considered for the market leaders: silver and copper, in comparison with the emerging field of aluminium inks.

Formulation of Screen-Printable Cu Molecular Ink for Conductive/Flexible/Solderable Cu Traces

Screen printing is the most common method used for the production of printed electronics. Formulating copper (Cu) inks that yield conductive fine features with oxidation and mechanical robustness on low-temperature substrates will open up opportunities to fabricate cost-effective devices. We have formulated a screen-printable Cu metal-organic decomposition (MOD) ink comprising Cu formate coordinated to 3-(diethylamino)-1,2-propanediol, a fractional amount of Cu nanoparticles (CuNPs), and a binder. This simple formulation enables ∼70-550 μm trace widths with excellent electrical [∼8-15 mΩ/□/mil or 20-38 μΩ·cm] and mechanical properties with submicron-thick traces obtained by intense pulse light (IPL) sintering on Kapton and poly(ethylene terephthalate) (PET) substrates. These traces are mechanically robust to flexing and creasing where less than 10% change in resistance is observed on Kapton and ∼20% change is observed on PET. Solderable Cu traces were obtained only with the combination of the Cu MOD precursor, CuNP, and polymer binder. Both thermally and IPL sintered traces showed shelf stability (<10% change in resistance) of over a month in ambient conditions and 10-70% relative humidity, suitable for day-to-day fabrication. To demonstrate utility, light-emitting diodes (LEDs) were directly soldered to IPL sintered Cu traces in a reflow oven without the need for a precious metal interlayer. The LEDs were functional not only during bending and creasing of the Cu traces but even after 180 min at 140 °C in ambient air without losing illumination intensity. High definition television antennas printed on Kapton and PET were found to perform well in the ultrahigh frequency region. Lastly, single-walled carbon nanotube-based thin-film transistors on a silicon wafer were fabricated with a screen-printed Cu source and drain electrodes, which performed comparably to silver electrodes with mobility values of 12-15 cm² V^-1 s^-1 and current on/off ratios of ∼10⁵ and as effective ammonia sensors providing parts per billion-level detection.

Sintering Copper Nanoparticles with Photonic Additive for Printed Conductive Patterns by Intense Pulsed Light

In this study, an ink formulation was developed to prepare conductive copper thin films with compact structure by using intense pulsed light (IPL) sintering. To improve inter-particle connections in the sintering process, a cuprous oxide shell was synthesized over copper nanoparticles (CuNP). This cuprous oxide shell can be reduced by IPL with the presence of a reductant and fused to form connection between large copper particles. However, the thermal yield stress after strong IPL sintering resulted in cracks of conductive copper film. Thus, a multiple pulse sintering with an off time of 2 s was needed to reach a low resistivity of 10^-5 Ω·cm. To increase the light absorption efficiency and to further decrease voids between CuNPs in the copper film, cupric oxide nanoparticles (CuONP) of 50 nm, were also added into ink. The results showed that these CuONPs can be reduced to copper with a single pulse IPL and fused with the surrounding CuNPs. With an optimal CuNP/CuONP weight ratio of 1/80, the copper film showed a lowest resistivity of 7 × 10^-5 Ω·cm, ~25% conductivity of bulk copper, with a single sintering energy at 3.08 J/cm². The ink can be printed on flexible substrates as conductive tracks and the resistance remained nearly the same after 10,000 bending cycles.

Ultrahigh Conductivity and Superior Interfacial Adhesion of a Nanostructured, Photonic-Sintered Copper Membrane for Printed Flexible Hybrid Electronics

Inkjet-printed electronics using metal particles typically lack electrical conductivity and interfacial adhesion with an underlying substrate. To address the inherent issues of printed materials, this Research Article introduces advanced materials and processing methodologies. Enhanced adhesion of the inkjet-printed copper (Cu) on a flexible polyimide film is achieved by using a new surface modification technique, a nanostructured self-assembled monolayer (SAM) of (3-mercaptopropyl)trimethoxysilane. A standardized adhesion test reveals the superior adhesion strength (1192.27 N/m) of printed Cu on the polymer film, while maintaining extreme mechanical flexibility proven by 100 000 bending cycles. In addition to the increased adhesion, the nanostructured SAM treatment on printed Cu prevents formation of native oxide layers. The combination of the newly synthesized Cu ink and associated sintering technique with an intense pulsed ultraviolet and visible light absorption enables ultrahigh conductivity of printed Cu (2.3 × 10^-6 Ω·cm), which is the highest electrical conductivity reported to date. The comprehensive materials engineering technologies offer highly reliable printing of Cu patterns for immediate use in wearable flexible hybrid electronics. In vivo demonstration of printed, skin-conformal Cu electrodes indicates a very low skin-electrode impedance (<50 kΩ) without a conductive gel and successfully measures three types of biopotentials, including electrocardiograms, electromyograms, and electrooculograms.

Filtration-induced production of conductive/robust Cu films on cellulose paper by low-temperature sintering in air

Cellulose paper is an attractive substrate for paper electronics because of its advantages of flexibility, biodegradability, easy incorporation into composites, low cost and eco-friendliness. However, the micrometre-sized pores of cellulose paper make robust/conductive films difficult to deposit onto its surface from metal-nanoparticle-based inks. We developed a Cu-based composite ink to deposit conductive Cu films onto cellulose paper via low-temperature sintering in air. The Cu-based inks consisted of a metallo-organic decomposition ink and formic-acid-treated Cu flakes. The composite ink was heated in air at 100°C for only 15 s to give a conductive Cu film (7 × 10^-5 Ω cm) on the cellulose paper. Filtration of the Cu-based composite ink accumulated Cu flakes on the paper, which enabled formation of a sintered Cu film with few defects. A strategy was developed to enhance the bending stability of the sintered Cu films on paper substrates using polyvinylpyrrolidone-modified Cu flakes and amine-modified paper. The resistance of the Cu films increased only 1.3-fold and 1.1-fold after 1000 bending cycles at bending radii of 5 mm and 15 mm, respectively. The results of this study provide an approach to increasing the bending stability of Cu films on cellulose paper.

Chemical formation of soft metal electrodes for flexible and wearable electronics

Efficient chemical approaches to fabricating soft metal electrodes aiming at wearable electronics are summarized and reviewed.