The afternoon of the first day of the IDTechEx Printed Electronics Asia event this week continued to illustrate how an increasing minority of developers of printed transistors target metal oxide versions with better performance. However, organic versions retain advantages of being tightly rollable and low voltage.
Toro Okubo of Toppan Printing Japan discussed developments in organic thin film transistors, though his company also develops zinc oxide transistors and has a partnership to produce nanosilicon transistors - the only company in the world to be serious about all three options.
Although most developers of OTFTs use ink jet, he employs a variety of printing methods, just as PolyIC does in Europe, though not in the same way.
Nano silver is used for offset printing of conductors. Flexo is preferred for the semiconductor layer. Unusually, Toppan has developed a special flexo printing technology suitable for small molecule pentacene, previously thought to be unprintable. This gives a slightly better transistor, with semiconductor mobility of around 0.72 cm2/vs. Here an extra benefit is improved reproduceability. Some ink jet and spin coating have also been employed.TFT backplanes for flexible electrophoretic displays are prepared.
Dai Nippon Printing
Dai Nippon Printing also targets active matrix flexible display drive circuits but Dr Hiroki Maeda said that gravure is preferred. Flexible OLED signage has been demonstrated but electrophoretics are now the main focus. These can be switched at 30Hz using liquid crystal organic semiconductors in its backplane driver transistors.
Fujitsu, NEC and Vorbeck Materials then presented their carbon nanotube and graphene capabilities. Fujitsu can make a forest of vertical multi wall carbon nanotubes topped by flat graphene sheets. Nano silver electrodes are used that cure at under 200C. Dr Sintaro Sato speculated that uses can include heat dissipation for high power transistors and interconnects on large scale integrated circuits. Future improvements must include better purity and avoiding the need to transfer print these assemblies.
By contrast, NEC of Japan in the form of Hiroyuki Endo, works on transfer printing a CNT based semiconductor on polyimide with the transistor dielectric being polyimide. NEC discovered carbon nanotubes in the first place. However, although the first transistors had only 1000 on-off ratio, he has achieved a remarkable10.3GHz performance with 150 microns channel length - a record.
Some of the resulting transistors should exhibit mobility of 1000 cm2/vs despite first ones being down at about one thousandth of that. Vorbeck Materials finished by looking at the graphene technologies and market vision.
For more read Carbon Nanotubes and Graphene for Electronics Applications and Inorganic and Composite Printed Electronics 2009-2019.