Moiré-fringeless Transparent Conductive Films with a Random Serpentine Network of Medium-Field Electrospun, Chemically Annealed Silver Microfibres.

Low-cost flexible transparent conductive films (TCFs) with direct writing of metal grids have been explored as a promising alternative to conventional indium-tin-oxide-based TCFs for future flexible electronics. However, flexible TCFs have raised technical concerns because of their disadvantages, such as low resolution, low productivity, poor optoelectrical performance, poor thermal stability, and adverse moiré fringes, which primarily arise from the superposition of periodic patterns. Herein, a facile and highly productive method to fabricate moiré-fringeless TCFs with good optoelectrical characteristics and excellent thermal stability is presented using a single-pass printed random serpentine network of medium-field electrospun silver microfibres (AgMFs) with a line width of 2.32 ± 0.97 µm by exploiting the random serpentine motion of medium-field electrospinning, enabling moiré-fringeless TCFs. The electrical in-plane anisotropy of the TCFs can be kept well below 110.44 ± 1.26% with the in situ junction formation of the AgMFs in the transverse direction. Combined thermal and chemical annealing of the AgMFs enables high productivity by reducing the thermal annealing time by 40%. The good optoelectrical performance, fair electrical in-plane anisotropy, high productivity, and superior thermal stability of the TCFs with the single-pass printed random serpentine network of medium-field electrospun AgMFs are suitable properties for flexible electronics such as ultra-large digital signage with LEDs.

Rapid jetting status inspection and accurate droplet volume measurement for a piezo drop-on-demand inkjet print head using a scanning mirror for display applications.

Despite the inherent fabrication simplicity of piezo drop-on-demand inkjet printing, the non-uniform deposition of colourants or electroluminescent organic materials leads to faulty display products, and hence, the importance of rapid jetting status inspection and accurate droplet volume measurement increases from a process perspective. In this work, various jetting status inspections and droplet volume measurement methods are reviewed by discussing their advantages and disadvantages, and then, the opportunities for the developed prototype with a scanning mirror are explored. This work demonstrates that jetting status inspection of 384 fictitious droplets can be performed within 17 s with maximum and minimum measurement accuracies of 0.2 ± 0.5 µm for the fictitious droplets of 50 µm in diameter and -1.2 ± 0.3 µm for the fictitious droplets of 30 µm in diameter, respectively. In addition to the new design of an inkjet monitoring instrument with a scanning mirror, two novel methods to accurately measure the droplet volume by amplifying a minute droplet volume difference and then converting to other physical properties are suggested and the droplet volume difference of ±0.3% is demonstrated to be discernible using numerical simulations, even with the low measurement accuracy of 1 µm. When the fact is considered that the conventional vision-based method with a CCD camera requires the optical measurement accuracy less than 25 nm to measure the volume of an in-flight droplet in the nominal diameter of 50 µm at the same volume measurement accuracy, the suggested method with the developed prototype offers a whole new opportunity to inkjet printing for display applications.

Electrostatic-Force-Assisted Dispensing Printing to Construct High-Aspect-Ratio of 0.79 Electrodes on a Textured Surface with Improved Adhesion and Contact Resistivity.

As a novel route to construct fine and abnormally high-aspect-ratio electrodes with excellent adhesion and reduced contact resistivity on a textured surface, an electrostatic-force-assisted dispensing printing technique is reported and compared with conventional dispensing and electrohydrodynamic jet printing techniques. The electrostatic force applied between a silver paste and the textured surface of a crystalline silicon solar cell wafer significantly improves the physical adhesion of the electrodes, whereas those fabricated using a conventional dispensing printing technique peel off with a silver paste containing 2 wt% of a fluorosurfactant. Moreover, the contact resistivity and dimensionless deviation of total resistance are significantly reduced from 2.19 ± 1.53 mΩ · cm(2) to 0.98 ± 0.92 mΩ · cm(2) and from 0.10 to 0.03, respectively. By utilizing electrodes with an abnormally high-aspect-ratio of 0.79 (the measured thickness and width are 30.4 µm and 38.3 µm, respectively), the cell efficiency is 17.2% on a polycrystalline silicon solar cell with an emitter sheet resistance of 60 Ω/sq. This cell efficiency is considerably higher than previously reported values obtained using a conventional electrohydrodynamic jet printing technique, by +0.48-3.5%p.

Contact resistivity decrease at a metal/semiconductor interface by a solid-to-liquid phase transitional metallo-organic silver.

We present a new approach to ensure the low contact resistivity of a silver paste at a metal/semiconductor interface over a broad range of peak firing temperatures by using a solid-to-liquid phase transitional metallo-organic silver, that is, silver neodecanoate. Silver nanoclusters, thermally derived from silver neodecanoate, are readily dissolved into the melt of metal oxide glass frit even at low temperatures, at which point the molten metal oxide glass frit lacks the dissociation capability of bulk silver into Ag(+) ions. In the presence of O(2-) ions in the melt of metal oxide glass frit, the redox reaction from Ag(+) to Ag(0) augments the noble-metal-assisted etching capability to remove the passivation layer of silicon nitride. Moreover, during the cooling stage, the nucleated silver atoms enrich the content of silver nanocolloids in the solidified metal oxide glass layer. The resulting contact resistivity of silver paste with silver neodecanoate at the metal/semiconductor interface thus remains low-between 4.12 and 16.08 mΩ cm(2)-whereas without silver neodecanoate, the paste exhibits a contact resistivity between 2.61 and 72.38 mΩ cm(2) in the range of peak firing temperatures from 750 to 810 °C. The advantage of using silver neodecanoate in silver paste becomes evident in that contact resistivity remains low over the broad range of peak firing temperatures, thus providing greater flexibility with respect to the firing temperature required in silicon solar cell applications.

The exothermic reaction route of a self-heatable conductive ink for rapid processable printed electronics.

We report the exothermic reaction route and new capability of a self-heatable conductive ink (Ag2O and silver 2,2-dimethyloctanoate) in order to achieve both a low sintering temperature and electrical resistivity within a short sintering time for flexible printed electronics and display appliances. Unlike conventional conductive ink, which requires a costly external heating instrument for rapid sintering, self-heatable conductive ink by itself is capable of generating heat as high as 312 °C when its exothermic reaction is triggered at a temperature of 180 °C. This intensive exothermic reaction is found to result from the recursive reaction of the 2,2-dimethyloctanoate anion, which is thermally dissociated from silver 2,2-dimethyloctanoate, with silver oxide microparticles. Through this recursive reaction, a massive number of silver atoms are supplied from silver oxide microparticles, and the nucleation of silver atoms and the fusion of silver nanoparticles become the major source of heat. This exothermic reaction eventually realizes the electrical resistivity of self-heatable conductive ink as low as 27.5 µΩ cm within just 40 s by combining chemical annealing, which makes it suitable for the roll-to-roll printable electronics such as a flexible touch screen panel.

Rapid two-step metallization through physicochemical conversion of Ag2O for printed "black" transparent conductive films.

A rapid two-step metallization for fabrication of a "black" transparent conductive film on a flexible substrate for display applications is presented, using a mixture of silver oxide (Ag2O) and silver neodecanoate (C10H19AgO2), and its electrical conductivity and colour transition behaviours are investigated. Silver nanoparticles, which are physicochemically converted from silver oxide microparticles in the presence of silver neodecanoate in the course of the first metallization step at 150 °C for 10 min, are chemically annealed by immersing them in an acidic ferric chloride (FeCl3) solution at room temperature for 10 s. During this second metallization step, silver nanoparticles are found to be tightly packed through Ostwald ripening, which eventually leads to the dramatic enhancement of electrical conductivity by six orders of magnitude from 1.33 S m(-1) to 1.0 × 10(7) S m(-1), which corresponds to 15.9% of the electrical conductivity of bulk silver. In addition to the enhancement of electrical conductivity, the silver chloride (AgCl) layer formed on the surface of the silver layer due to ferric ions (Fe(3+)) enhances the blackness of the transparent conductive film by a factor of 1.69, from 36.29 B to 61.51 B. The sheet resistance and optical transparency of a roll-to-roll printed black transparent conductive film for a touch screen panel are found to be as low as 0.9 Ω□(-1) and 81%, respectively, after conducting the proposed two-step metallization.

Printed UHF RFID antennas with high efficiencies using nano-particle silver ink.

One of the most popular targets of conductive ink technology is to print RFID tag antennas. However, the printed RFID antennas, manufactured by conductive silver ink which is generally based on microsized silver particles, have lower conductivity and consequently lower radiation efficiency than those by conventional copper etching method. This work demonstrates nano-particle conductive silver ink that is capable of printing UHF RFID antennas with improved radiation efficiency. Compared with commercial micro-particle silver ink, the solid content of metal is much higher in the proposed nanoparticle silver ink, leading to better electrical properties. Two types of dipole antennas are printed with the proposed nano-particle as well as with commercial micro-particle inks. Also, the same antennas are fabricated by copper etching. With these conductive inks, a straight and a meandered dipole antennas are fabricated and their radiation efficiencies are measured with the Wheeler cap method. Experimental results show that the radiation efficiencies of the antennas based on nanoparticle silver ink are superior to those printed with the micro-particle silver ink, and are comparable to those of popular copper antennas.

Self-patterning of fine metal electrodes by means of the formation of isolated silver nanoclusters embedded in polyaniline.

A novel self-patterning technique, which is not based on surface energy differentiation with low or high surface energy as used in conventional surface energy techniques, is presented, using polyaniline and an organometallic silver compound. The underlying mechanisms used to control the electrical conductivity of the metallic silver converted from the organometallic silver compound by a thermal process are also analysed, with electrical and microstructural characterizations. It is found that polyaniline in conjunction with the organometallic silver compound changes its oxidation state from the conductive emeraldine form to the non-conductive pernigraniline form, and silver nanoclusters in the presence of polyaniline do not form a continuous conductive network, although the sheet resistance of silver nanoclusters in the absence of polyaniline is as low as 4.27 +/- Omega/ square at the curing temperature of 230 degrees C. By means of these chemical and physical traits of polyaniline and the organometallic silver compound, alternating conductive and non-conductive stripes were self-patterned without the aid of surface energy patterning techniques, the linewidth and nominal pitch of which went down to 23.2 +/- 0.7 microm and 40 microm, respectively.

Oscillatory limited compressible fluid flow induced by the radial motion of a thick-walled piezoelectric tube.

A simple oscillatory, slightly compressible, fluid flow model in a thick-walled piezoelectric tube used in a drop-on-demand inkjet print head is developed from the point of view of fluid-structure interaction to take account of pressure wave propagation and pressure loading opposing wall motion. A frequency sweep is performed computationally using the model revealing the first acoustic fluid-structure resonance frequency and the influence of fluid viscosity. The validity of the model, with given information on the speed of sound in a fluid, is evaluated by comparing the theoretically predicted resonance frequency to the experimentally measured resonance frequency. In addition, the intrinsic speed of sound can be easily computed using the measured acoustic resonance frequency and this computed speed of sound agrees closely with speeds of sound reported in the literature.