The properties of organic electronics allow them to be processed at low temperatures onto flexible substrates, opening up a new and novel field of low
-cost high throughput electronics. Up to date, research into the roll-to-roll printing of electronics has concentrated on the use of solution based methods for the deposition of material. That is, the use of solvents, which
presents a number of problems. Here the methods used for the deposition of materials were compatible with vacuum deposition processes, avoiding the problems that are encountered with the use of solvents.
First transistors were fabricated using naphtho[2,3-b]naphtho[2′,3′:4,5]thieno[2,3-d]thiophene (DNTT) semiconductor on Si/SiO2 substrates to determine if recrystallization alone was a suitable method for the purification of DNTT when compared to sublimation. It was found by transistor transfer measurements that although recrystallized DNTT gave a larger spread in mobilities than sublimated DNTT, it had a higher average saturation mobility, 0.54 cm2/Vs compared to 0.36 cm2/Vs. Two transistor configurations were then investigated on poly(ethylene 2,6-naphthalate)(PEN)
substrates with polystyrene (PS) dielectric and recrystallized DNTT semiconductor. The performance of top-gate-bottom-contact (TGBC) and bottom-gate-top-contact (BGTC) transistors was evaluated by output and transfer measurements. It was found that the mobilities of the BGTC transistors were significantly higher, an average of 1.01 cm2/Vs in the
linear regime and 0.97 cm2/Vs in the saturation regime, than those of the TGBC transistors, ~0.05 cm2/Vs in the linear regime and ~0.016 cm2/Vs in the saturation regime. However, despite the high mobility the transistor yield was unacceptably low, at best~ 65 %. Although the mobilities of the TGBC transistors were relatively low, it was still possible using that
configuration to fabricate inverters that had a gain in excess of 1 for a number of rail voltages, (-60 V, -40 V, -20 V = VDD). BGTC Transistors were then fabricated on tri(propyleneglycol) diacrylate (TPGDA) dielectric
with recrystallized DNTT semiconductor. The average mobility in the saturation regime was found to be 0.44 cm2/Vs, lower than for the PS dielectric transistors, and had a dependence on channel width, W, due to device design. However, the device yield was 89%, an improvement on the solution processed device yield. Although the device yield was good, the stability of the devices made it difficult to fabricate inverters with stable operation. Due to the device instability it was not feasible to fabricate more complicated devices.
By using a PS buffer layer to improve the surface of TPGDA, transistors with an average saturation mobility of 1.51 cm2/Vs were fabricated with a yield of 90%. Here again the mobility was shown to be W dependent, although not to the same degree as for TPGDA/DNTT transistors. Transistors that had been stored under lab conditions for six months in the dark were shown to still have good mobility and stable device operation with no hysteresis. Using the same device configuration it was possible to fabricate working, stable inverters with high switching speeds with total rise and fall times of less than 1 ms. Ring oscillators were also fabricated with output frequencies in the low KHz range, in excess of previously published R2R printed ring oscillators. Functioning NAND and NOR gates were also fabricated with similar switching speeds as the inverters. Finally, a working fully-integrated NAND-based SR Flip-flop gate was also fabricated.