Inkjet printing of pixel arrays: droplet formation and pattern uniformity of a non-aqueous ink with tunable viscosity

Abstract

Inkjet printing is a rapid and material-efficient process that is suitable for the fabrication of large-area microarrays from a range of optoelectronic materials. In order to ensure stable droplet formation and a uniform print image with very smooth surfaces, however, the ink properties such as viscosity and surface tension have to be precisely adjusted. In this study, a non-aqueous ink formulation is proposed whose viscosity can be conveniently adjusted by controlling the mixing ratio of propylene carbonate (PC) as the low-viscosity solvent and propylene glycol (PG) as the high-viscosity solvent. Using a combination of advanced imaging techniques, we show that raising the PG content from 20% to 80% increased the viscosity of the ink from 3.36 cP to 26.70 cP, resulting in stable droplet formation and a more evenly printed image. At a spacing of 5 dots/pixel, the roughness value decreased dramatically, from root mean square (RMS): 11.28 (20% PG) to RMS: 0.09 (80% PG). Alternatively, more homogeneous patterns (albeit with a rough surface) were also produced with the low-viscosity ink (20% PG) when a conditioned substrate with low surface energy and selective liquid repellency was used. With this we present a simple but effective strategy to improve droplet formation while obtaining highly uniform pixel arrays. The knowledge gained will be particularly useful for inkjet printing of pixel-patterned color conversion layers in devices such as organic light-emitting diodes (LEDs) and micro-LED displays.