Electrohydrodynamic (EHD) Jet Technique for Indium-Zinc-Oxide (IZO) Thin-Film Transistors

Many studies have reported that oxide TFTs for printed electronics can be processed using solution methods. Printed electronics can circumvent time consuming-, expensive-, and chemical waste-photolithography process for patterning. One of the issues in solution processed oxide TFTs is that a high temperature annealing process is needed to remove organic residues because they can deteriorate the electrical characteristics. For a low temperature process, many studies have examined a variety ways to reduce the reaction under 200^oC but retain excellent performance. Another issue is to apply a printing process for fine patterning. Most studies of solution-processed oxide semiconductors were carried out by spin coating or ink-jet printing. The electrohydrodynamic (EHD)-jet printing process is unique technique that can overcome these limits. In the case of a paste, transpiring is possible by setting a high voltage because inks are forced down by the electric field between the nozzle and substrate. The ink can form a drop to spray configuration under a range of bias voltages.  Electrohydrodynamic (EHD) jet-printed indium- zinc-oxide thin-films transistors (TFTs) were prepared at various annealing temperatures. The EHD jet was achieved by optimizing the process parameters; such as the precursor viscosity, driving voltages, and substrate temperatures. The EHD jet-printed IZO films were characterized using a range of analytical methods. Metal oxide formation was confirmed by X-ray photoelectron spectroscopy. The EHD jet-printed IZO TFTs showed good electrical properties: mobility of 4.8 cm²/Vs, threshold voltage of 8.4 V, on-to-off current ratio of 10⁷, and a subthreshold slope of 1.2 V/dec at 400^oC. The positive and negative bias stability were also analyzed.