A Study of the Preparation and Properties of Antioxidative Copper Inks with High Electrical Conductivity
© Tsai et al. 2015
Received: 20 July 2015
Accepted: 2 September 2015
Published: 15 September 2015
Conductive ink using copper nanoparticles has attracted much attention in the printed electronics industry because of its low cost and high electrical conductivity. However, the problem of easy oxidation under heat and humidity conditions for copper material limits the wide applications. In this study, antioxidative copper inks were prepared by dispersing the nanoparticles in the solution, and then conductive copper films can be obtained after calcining the copper ink at 250 °C in nitrogen atmosphere for 30 min. A low sheet resistance of 47.6 mΩ/□ for the copper film was measured by using the four-point probe method. Importantly, we experimentally demonstrate that the electrical conductivity of copper films can be improved by increasing the calcination temperature. In addition, these highly conductive copper films can be placed in an atmospheric environment for more than 6 months without the oxidation phenomenon, which was verified by energy-dispersive X-ray spectroscopy (EDS). These observations strongly show that our conductive copper ink features high antioxidant properties and long-term stability and has a great potential for many printed electronics applications, such as flexible display systems, sensors, photovoltaic cells, and radio frequency identification.
In the past few years, conductive inks have attracted considerable attention due to their growing application in electrodes of silicon-crystal solar cells  and the printed electronics industry, such as smart labels , flexible displays [3, 4], and radio frequency identification (RFID) [5, 6]. Currently, silver inks have been commonly developed to enable outstanding conductivity and excellent printability [7, 8]. However, the high price and scarcity of such material limit wide industrial applications . In addition, the low dispersion stability of silver ink would cause particles to aggregate, which could lower the quality of the silver thin film easily. In view of these, copper is a good alternative material for silver because of its high electrical conductivity and low price. These advantages could be highly beneficial for the reduction of the manufacturing cost. Nevertheless, a fundamental problem with copper material is its high susceptibility to oxidation in an atmospheric environment [10, 11]. Thus, the prevention of oxidation and high stability for copper inks are crucial issues to prepare conductive copper ink.
For the fabrication method of copper nanoparticles, hydrazine reductants are mostly adopted in early times . Unfortunately, this process is highly toxic and dangerous, which could cause serious pollution and put people at risk of exposure when the samples are produced. However, if sodium borohydride  or sodium hydrophosphate  is utilized as the reductant, impure substances would be difficult to purify, or the synthesis needs to be performed in a vacuum environment, which leads to the cost increment. Therefore, various novel methods for synthesizing copper nanoparticles are gradually developed. For instance, copper hydroxide and l-ascorbic acid are used as the precursor salt and the reductant, respectively . Such wet chemical reduction method has the advantage of avoiding toxic materials. In addition, the oxidation of products could be further prevented by using polymeric protectors. For the preparation method of conductive copper ink, a specific solvent and dispersant are very critical to disperse copper nanoparticles uniformly in the solvent and prevent aggregation of copper nanoparticles, which would influence the electrical conductivity and antioxidative ability of the copper nanoparticles. If the copper ink is oxidized easily, the quality of the silicon solar cells and the printable electronic materials would be inferior. Even if oxidation occurs after the electrodes are formed, the rapid increase in the resistance of the electrodes would reduce severely the power-generating efficiency of the silicon solar cells and affect the electrical conductivity of the printable electronic materials such as the printed circuit board (PCB). Thus, it is required to provide a method for preparing conductive copper ink having a high oxidation resistance and superior dispersibility. In this work, we present the experimental method for preparing the conductive copper ink with high antioxidant properties and investigate the electrical performances of copper ink with different solvent proportions and calcination parameters. In particular, a very high antioxidative stability for our conductive copper ink is observed. This work can greatly facilitate our abilities to develop antioxidative copper inks applied in many practical applications. For instance, we can apply quickly these copper inks to repair PCB interconnect defects occurring in the manufacture of PCBs. Furthermore, copper ink is a good candidate to substitute for silver ink especially for application to electrodes of solar cells. Interestingly, copper ink is applicable to writing. We can use a roller pen filled with copper ink to design a series of copper patterns, such as lines, electrodes, and RFID antennas. This is an easy and promising fabrication process to make conductive patterns for portable applications where the pattern is required.
Synthesis of Antioxidant Copper Nanoparticles
In this study, we have developed a simple method that features the all-solution processes in a non-vacuum environment and is low-pollution, low-cost, and less time-consuming (<1 h) for the synthesis of antioxidant copper nanoparticles so far. Antioxidant copper nanoparticles were synthesized using the wet chemical reduction method. Cu(OH)2 and PVP were dissolved in ethylene glycol solution. The solution was stirred with a magnetic stirrer for 30 min to ensure that the Cu(OH)2 and PVP were dissolved completely. Then, an ascorbic acid polyol solution was dissolved under the same conditions. The latter solution was poured into the former flask, and the color of the solution turned from blue to brown within 5–10 min, indicating the formation of copper nanoparticles. Finally, the resulting dispersion was washed with ethanol at 5000 rpm for 5 min via centrifugation.
Preparation of Antioxidative Conductive Copper Ink
Characterization and Measurements
The morphologies and structures of the copper nanoparticles and conductive films were characterized by field-emission scanning electron microscopy (FE-SEM, JSM-7100F). In addition, we inspected the elemental composition and distribution of conductive copper films through energy-dispersive X-ray spectroscopy (EDS) analysis by using the Oxford X-Max8 equipped in the FE-SEM. For electrical characteristics of the conductive film, we utilized the Napson RT-7 four-point probe resistance measurement system to measure sheet resistance.
Results and Discussion
The sheet resistances of copper films with different solvent proportions of copper inks
Solvent proportion (water-free alcohol/tert-butanol)
Sheet resistance (mΩ/□)
The sheet resistances of copper films with different calcination parameters
Calcination temperature (°C)
Sheet resistance (mΩ/□)
In summary, we have provided a method for preparing antioxidant conductive copper ink and investigate the characteristics of copper films with different solvent proportions and calcination parameters. The ratio of the weight percentage of water-free alcohol to that of tert-butanol is preferably 1:2. For electrical resistivity of the copper film, a low sheet resistance of 47.6 mΩ/□ is achieved after calcining the copper ink at 250 °C in nitrogen atmosphere for 30 min. In addition, we experimentally demonstrate that the electrical conductivity of copper films can be improved by increasing the calcination temperature. Significantly, our conductive copper film can be placed at room temperature for more than 6 months without the oxidation phenomenon. These unique features of our antioxidative conductive copper ink are particularly useful to many printed electronics applications such as flexible display systems, sensors, photovoltaic cells, and radio frequency identification.
This work was supported in part by the Ministry of Science and Technology of Taiwan, under grant number MOST 104-31111-Y-042A-056. The authors earnestly appreciate the Institute of Nuclear Energy Research (INER) for all the technical assistance concerned with this work.
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