Direct printing of functional electronic materials may provide a promising route to low-cost fabrication of integrated circuits. From this point of view, ink-jet printing has received special attention as a direct patterning technique for the cost-effective fabrication of organic electronic devices such as organic thin-film transistors (OTFTs) (Figure 1) and organic photovoltaic cells.

Professor Kilwon Cho and Ms. Jung Ah Lim of the Department of Chemical Engineering have succeeded in fabricating high-performance OTFTs by inkjet printing of organic semiconductor. This result entitled “Self-Organization of Inkjet-Printed Triisopropylsilylethynyl Pentacene via Evaporation- Induced Flows in a Drying Droplet” was published and introduced in the January issue of the Advanced Functional Materials as the cover story.

To produce organic electronic devices with high performance via inkjet printing, the uniform deposition of organic semiconductor thin-film with desired molecular ordering by inkjet printing has become an essential challenge because charge carrier transport in organic electronic devices is strongly influenced by the crystalline microstructure and morphology of the organic semiconductor film.

However, the uneven distribution and random orientation of organic semiconductor molecules were commonly observed in most of organic semiconductor films printed from homo-solvent, which was disadvantageous to electrical property of the devices.

Prof. Cho’s research group reported for the first time that inkjet printing of organic semiconductor films with uniform morphology and a desired molecular orientation can be achieved by varying the composition of the solvent mixture. They found that self-aligned crystals of organic semiconductor with highly ordered crystalline structures can be inkjet printed in presence of the minor solvent, which has a higher boiling point and a lower surface tension than the major solvent (Figure 2). These self-aligned organic semiconductor crystals can be used successfully to produce highperformance organic transistors.

They illustrated that this approach makes use of the evaporation-induced flows, in particular the convective and Marangoni flows that occur in an inkjet printed droplet during drying process. The convective flow that transports the molecules in a droplet to the perimeter of droplet can be counterbalanced, depending on the solvent composition, by the Marangoni flow that is induced by the surface tension gradient between the periphery and the interior of the droplet from regions with low to regions with high surface tension.

They have confirmed that high performance inkjetprinted transistors can be obtained by optimizing the deposit morphology and crystalline structure of inkjet-printed organic semiconductor films by controlling the evaporation-induced flow in the printed droplets. This finding may offer an excellent way to control the molecular ordering of organic semiconductors for the direct-write fabrication of high-performance organic electronics. This development of printed electronics will realize new electronic products such as wearable electronics in garments, electronic toys in giveaways, or even electronic bar code on a yogurt cup in the near future.

Prof. Kilwon Cho (Dept. of Chemical Engineering) received his Ph.D in polymer science from the University of Akron in 1986, and worked as a researcher at IBM Research Center. He has taught at POSTECH since 1988.

Professor Kilwon Cho
Department of Chemical Engineering
Polymer Research Institute
Tel: +82-54-279-2270
Fax: +82-54-279-8298
E-mail: kwcho@postech.ac.kr