New N-type Organic Semiconductor Stable in Atmosphere

Sept 3, 2010 01:33 Tetsuo Nozawa, Nikkei Electronics

The Organic Electronics Research Group of the Kyushu Institute of Technology (KORG) developed an n-type organic semiconductor that shows a high stability in the atmosphere.

It can be made in a simple manufacturing process and at room temperature, the research group said. The development was announced at a preview meeting that took place prior to the 71th Autumn Meeting 2010 of the Japan Society of Applied Physics, which will run Sept 14 to 17, 2010, in Japan (lecture number: 15a-H-4).

KORG actually made an organic TFT (OTFT) by using the n-type organic semiconductor and confirmed that its performance hardly deteriorated for more than 119 days in the atmosphere with a high temperature and humidity, said Shuichi Nagamatsu, assistant professor at the Kyushu Institute of Technology.

Moreover, KORG formed a CMOS inverter circuit combined with a p-type organic semiconductor on a plastic film and confirmed that its performance hardly deteriorated for two weeks in the atmosphere.

While many p-type organic semiconductors with a high mobility and durability have already been developed, most n-type organic semiconductors require complicated manufacturing processes and have very short lives in the atmosphere. Because of the development of the new n-type organic semiconductor, the roll-to-roll production of organic RFIC tags and application to organic thin-film photovoltaic (PV) cells are now one step closer to realization, Nagamatsu said.

Being left at room temperature for 2 hours

The new n-type organic semiconductor is made by chemically reacting two types of commercially available benzene derivatives for two hours in alcohol with use of sodium ethoxide. The reaction, which is called Knoevenagel condensation, proceeds at room temperature, and the yield is as high as 93%.

When an OTFT was formed on a silicon substrate by using the n-type organic semiconductor, its carrier mobility and the ratio of on-current to off-current were 0.17cm²/Vs and 10⁶, respectively, in a vacuum, Nagamatsu said. After the OTFT was exposed in the atmosphere with a humidity of 60 to 85% for 120 days, the carrier mobility was more then 0.1cm²/Vs, he said.

Furthermore, when a CMOS inverter circuit was formed on a PEN film substrate by using the n-type organic semiconductor and dimethyl styryl benzene, which is a p-type semiconductor material known for its high stability in the atmosphere, it showed an inverter performance of 40 (value of gain) or more. And the performance hardly changed after the CMOS inverter circuit was left for two weeks in the air whose moisture was removed by desiccant.

For the future, KORG aims to improve the carrier mobility, which is about 90% lower than that required to realize organic RFIC tags, by developing new materials and improving device properties.

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