Organic semiconductors open path for flexible electronics


News & Trends • Posted:16 October 2013

mix-id328072.jpgScientists from the Emergent Molecular Function Research Group at the RIKEN Centre for Emergent Matter Science have developed a synthetic procedure that according to them makes it easier to tailor the chemical structure of an important organic semiconductor.

 

Organic semiconductors made from small aromatic molecules can be dissolved and screen-printed onto many substrates, including plastics, opening the path for ‘flexible’ electronic devices such as low-cost polymer solar cells.

Kazuo Takimiya and colleagues, in collaboration with researchers from Hiroshima University, have now developed a synthetic procedure that makes it easier to tailor the chemical structure of an important organic semiconductor.

Kazuo Takimiya and his team, in collaboration with researchers from Hiroshima University, were studying molecules known as diacene-fused thienothiophenes when they discovered their new synthetic procedure. Diacene-fused thienothiophenes are composed of interlocking benzene and sulphur-containing aromatic rings and are more durable, and have higher charge carrier mobilities, than most other organic semiconductors. Although current schemes to make these compounds are relatively straightforward, they are also difficult to modify. Thus, chemists have a hard time producing derivatives based on this ring system with more desirable properties.

The researchers devised a creative synthesis that, instead of relying on bulky aromatic precursors, generates diacene-fused thienothiophenes from small molecules through two consecutive ring-forming reactions. First, they generated an active reagent called phenylsulfenyl chloride that joins to a benzene–acetylene molecule and transforms it into a three ring system. Then, they used selective carbon–hydrogen bond activation to set off a rare intramolecular coupling that produces a molecule with four fused rings known as benzothieno-benzothiophene (BTBT). Takimiya explained that this approach produces excellent yields and makes it possible to scrutinise numerous BTBT derivatives by making simple changes to the starting reagents.

Trials revealed that this technique was particularly useful for extending the ring structure of BTBT-type molecules. For example, by substituting double- and triple-fused benzene molecules into the synthetic procedure, the team linearly constructed the BTBT substructure to form five, six and seven aromatic rings. Intriguingly, these new derivatives have an asymmetric structure that may dramatically improve their solubility—an important processing feature for printed electronics and one that is difficult to achieve using existing synthetic techniques.

Lengthening the BTBT framework to an eight-ringed symmetric structure also yielded a potent new organic semiconductor with excellent thermal stability and a charge carrier mobility five times higher than that of BTBT. “This mobility is among the highest recorded for thin film organic field-effect transistors, meaning that this molecule could be a candidate for real flexible electronics applications in the future,” stated Takimiya.

Solution coating the easy way


201306047919620Researchers in the US and China have developed the first solution-coating technique capable of producing high-quality, large-area single-crystalline organic semiconductor thin films suitable for high-performance, low power and inexpensive printed electronic circuits. The technique, dubbed FLUENCE (fluid-enhanced crystal engineering) can be used to make thin film organic semiconductors with record charge carrier mobilities.

Fluid flow around micropillars

Solution coating of organic semiconductors is an excellent method for making large-area and flexible electronic materials. However, it is not at all good for making aligned single-crystalline thin films – the ideal form for organic semiconductors and that have the best electronic properties. Aligned crystals are preferred in these materials because charge carrier transport through these structures depends on the crystal orientation.

A team led by Zhenan Bao at Stanford University and Stefan Mannsfeld of the SLAC National Accelerator Laboratory is now reporting on a new solution-coating method that can produce high-quality, millimetre-wide and centimetre-long highly aligned single-crystalline organic semiconductor thin films. The essence of FLUENCE is that we are able to control the flow of liquid in which the organic semiconductor is dissolved, explains team member Ying Diao. During fast printing, this “ink” often distributes itself unevenly – something that leads to defects and other structural imperfections quickly appearing in the semiconducting crystals.

FLUENCE tackles this problem from two angles, she says. First, it works using a microstructured printing blade containing tiny pillars that mixes the ink uniformly. Second, specially designed chemical patterns on the substrate prevent the crystals from aligning randomly or “stochastically” in a direction that would be the opposite to that in which printing is taking place. These two methods combined lead to large-area highly aligned single crystalline films that are much more structurally perfect.

To prove that their technique works, the researchers fabricated an organic semiconductor made from TIPS-pentacene, a routinely used and much studied organic semiconductor material, and found a charge carrier mobility of as high as 11 cm2 V−1 s−1. This is the first time a mobility of greater than 10 cm2 V−1 s−1 has been reported for TIPS-pentacene.

“The concepts we have developed in FLUENCE could easily be scaled up and applied to commercial printing methods,” said Diao. “The significant improvement in structural quality and electrical performance of the thin films printed with our method could allow to make higher performance, lower power, small and inexpensive organic circuitry,” she told nanotechweb.org. “We hope that our work will help advance such a morphology-by-design approach to make organic semiconductors for high-performance, large-area printed electronics.”

The team, which includes researchers from Nanjing University in China, says that it will now look at pattering aligned crystals at length scales suitable for making sub-micron devices.

The present work is reported in Nature Materials.

About the author

Belle Dumé is contributing editor at nanotechweb.org.