Traditionally transparent electrodes were made out of metal oxides at high temperatures inside vacuum chambers. These oxides made the transparent electrodes brittle and along with
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the cost of fabrication within a vacuum led to the pursuit of cheaper more efficient substitute methods. Studies in [61] proposed an alternative method for the manufacturing of transparent electrodes which are vital components of most optoelectronic devices. They fabricated a transparent electrode by EHD printing a fine grid measuring less than 10µm in width out of silver nanoparticle ink. The gap between consecutive grid lines was varied to study the effect of gap distance on the resistance and optical properties of the printed grid. They identified that a line to line gap distance of 150µm was necessary for an appropriate compromise to be reached between good optical properties and favourable electrical properties for the sheets of transparent electrodes that were printed.
The optical transmittance results revealed that as the line-to-line gap reduced from 300µm to 50µm the transmittance fell from 86% to 67% at a single visible wavelength of 550nm. Nevertheless at gap distances of 150µm the transmittance values were consistently at 80%. The printed silver sheets were 8.9µm thick and had a resistivity of 78.2μΩcm (4.87Ωsq−1). The also authors noted that predicted optical transmittance values and electrical resistance values for the printed transparent electrodes digressed slightly from measured values due to minor variations in width (7.5µm) and height (1.46μm) of the silver lines printed which was attributed to the electrohydrodynamic technique being a liquid based technology. Transparent grids were also printed with the piezoelectric inkjet printing technique to illustrate the clear advantage the electrohydrodynamic printing technique has over traditional patterning techniques. The grids had a line width of 50µm and were observable to the human eye which was in contrast to the less than 10µm lines produced by the EHD technique.
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Similar work was attempted [139] with silver ink and invisible metal meshes for electromagnetic shielding purposes. They reported similar printed line widths of less than 10µm (9.72µm) with a printed line height of (1.04µm). Yet the difference in their work lay in the optimum line to line gap distance of 300µm which is twice as much when compared to the proposed 150µm by [61]. Their silver meshes also had optical transmittance values that were slightly higher at (88%) while those printed by [61] were capable of an optimum of 80%. The electrical resistance properties of the silver meshes printed by [139] were also dissimilar at reported values of less than 7Ωsq−1 while those of [61] were 4.87Ωsq−1. The difference also extended to the printing conditions which were markedly different for both studies. Novel work in [140] described a hybrid approach to the fabrication of transparent electrodes in optoelectronic devices where EHD jet printing was used along with brush painting. A silver grid (pitch=300µm) with a measured printed width of 7.5µm was rooted within a film of indium tin oxide nanoparticles (average particle size = 20nm) that was brush painted on. This resulted in a sheet resistance of (1.4Ω/square) with a resistivity of (4.2010-5 Ω-cm) while maintaining high values of optical transmittance at (83.72%).
The investigations [139] and [61] used similar materials (Ag 70% nanoparticle content printing ink) at a viscosity of 4300cP. The printing ink was pumped into the print head at a flow rate of 150 nl/min in both studies but [61] used a much larger printing head nozzle (150µm) while [61] used a smaller nozzle of 100µm. Despite the larger nozzles, [139] printed at low standoff heights of 1mm and at a low applied voltage of 1kV. In addition to thin printing nozzles, [61] operated at double the applied voltage of 2kV at a standoff height that was five times higher (5mm) than those used by [139]. The substrates that the silver meshes
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were printed on was also different for both studies, [139] printed their meshes on polyether sulfone (PES) while [61] printed on glass substrates.
High viscosity silver ink (10,000 mPas) containing 78.9wt% of silver nanoparticles was used by [61] to print 84 silver electrodes with a high aspect ratio for use in the manufacture of solar cells via a non-contact printing technique. The 20µm wide, 0.81µm high electrodes were printed in the continuous jet mode under an applied voltage of 1.8 kV and a flow rate of 0.5µl/min on the pure silicon wafer surface. To study the effect of surface energy and contact angle, printing was done on three different surfaces that included two hydrophobic surfaces 1) a silicon wafer with Hexamethyldisilazane treatment, 2) a silicon surface with Decyltrichlorosilane treatment and 3) a bare silicon wafer. They printed twice over the silver electrodes in an attempt to increase the height of the electrodes while keeping the width to a minimum. At a printing speed of 4000mm/min, the bi-fold printing resulted in an increase in line width from 20µm to 47.9µm with a 3.5 fold increase in the printing height to 2.9µm for the SW-DTS surface while the opposite was true for the SW-HDMS surface electrodes where the same height (2.9µm) was reported with three fold increase in width to 60.9µm. Despite the novel approach, the electrohydrodynamically printed solar cells had similar open circuit voltage (Voc) and short circuit current density (Jsc) values of 0.61, 32-33 mA/cm2 which is comparable to solar cells manufactured through conventional means. Moreover, the electrohydrodynamically printed solar cells behaved less efficiently.
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Findings in [141] acknowledged the failings in electrohydrodynamically printed solar cells and attributed it to the quality of the silver printing paste used which results in either a low packing density or a high contraction in printed volume. To investigate this they prepared a silver paste that consisted of two distinctly different silver particles sizes. The small particles measuring (0.13–0.35 μm) would permit an even flow of the silver paste through the printing nozzle but would hinder the effective contact between the silver electrodes and the emitter layer of the crystalline solar cell. To offset this, large silver particles measuring (0.9–1.4 μm) were uniformly mixed in to the paste. They were able to successfully print silver electrodes (60µm wide and 51.50µm high) with a height to width aspect ratio of 0.86 and reported a solar cell efficiency of 16.72% which is slightly more than the solar cells printed with diluted commercially available silver paste (13.7%). The emitter sheet had a resistance of 60 Ω/sq with open circuit voltage (Voc) and short circuit current density (Jsc) values of 616.8 mV and 34.61 mA/cm2.
2.15 Electrohydrodyanmic printing of field effect transistors, ferroelectric