Roll-to-Roll Printed TFT Backplanes

The 2014 Printed Electronics Conference included an interesting presentation by Prof. Gyoujin Cho, of Sunchon National University, Korea entitled, Fully Roll-to-Roll (R2R) Gravure Printed TFT Backplanes for Signage and Sensors. In describing his presentation Prof. Cho made reference to the term “Smart Wallpaper” which is how he envisions the goal of his research. Prof. Cho identified the central challenges confronting the manufacture of printed TFT backplanes as being frequency of operation, power consumption, threshold voltage shift, scalability and stability. He set goals for the performance figures of merit including operation to 100 kHz, threshold voltage operation at 20 VDC falling to about 1.5V, one year operational stability and improved (decreased) threshold voltage shift.

Prof. Cho described the use of silver ink for conductors, single wall carbon nanotube ink for a semiconductor material and barium titanate ink for use as a capacitor dielectric material which can be used in a relatively thick layer, while still yielding high capacitor values owing to its high dielectric constant. In his work, Prof. Cho used PEN (polyethylene naphthalate) as the substrate material.

To date, the Sunchon National University researchers have achieved thin film transistor (TFT) operation to 1 kHz for a gravure-printed TFT, a frequency that Prof. Cho said was high enough for low functionality devices. Roll-to-roll (R2R) gravure printed transistors were printed on a 150 meter long substrate in 20 minutes. This relatively high speed printing of the TFT transistors is the technological basis for Prof. Cho’s Smart Wallpaper concept.

Prof. Cho identified several key issues that were revealed during his research. The researchers were able to obtain threshold voltage variation less than 5%. Prof. Cho pointed out that the requirements to realize his proposed smart wallpaper were to obtain a semiconductor layer with mobility of 10-30 cm²/v-sec, a transistor on/off ratio 107-108, with a five year lifetime. Based on his prior data and the results of the present work, Prof. Cho has a clear roadmap in mind to realize smart wallpaper and he is making considerable progress toward this goal.

In his Printed Electronics 2014 presentation Prof. Cho went on to show the camera system that he employed for registration control of the roll-to-roll gravure printing process. He also presented results on a R2R printed 20×20 TFT backplane and illustrated the use of this smaller TFT array as a pressure sensor. All in all, Prof. Cho’s talk at Printed Electronic 2014 presented impressive research results for R2R-printed TFT transistor arrays with good intermediate results and a view toward a long term smart wallpaper roadmap. – Phil Wright