Self-assembly and top-down approaches combined for organic electronics fabrication

Researchers in the US have developed a surface treatment technique that may lead to a low-cost method for producing large arrays of organic transistors on polymer sheets. Potential applications include flexible displays, ‘intelligent paper’ and biosensor arrays for field diagnostics.

The process developed at the National Institute of Standards and Technology (NIST), the University of Kentucky and Penn State University is based on a chemical pre-treatment of electrical contacts that induces self-assembly of molecular crystals. This is said to improve the performance of organic semiconductor devices, and help provide electrical isolation between devices.

“These devices are not building themselves,” says Penn State engineer Tom Jackson, who developed the technique together with NIST’s David Gundlach, Kentucky chemist John Anthony and others. “We are using the electrode structure as a way to influence and deterministically control the self-assembly of the microstructure that we need. It’s an early attempt at connecting the device, structure and material together.”

Devices created using the new process have electron mobilities similar to, and in some cases better than, amorphous silicon. The next step is to improve the robustness of the devices and the reproducibility of the technique for real-world commercial applications.

Jackson describes the latest work as a next logical step in a progression led by organic electronics specialists who use surface energy to improve the resolution of their inkjet printing processes. “Now we’re not only localising the material,” says Jackson. “We are developing a structure at the molecular level which is directly tied to the device structure. We hope it’s an important step in convincing these materials to do what is needed in a way that provides a very cost-effective fabrication technology.”

Natalie Stingelin, who wrote a Nature News & Views commentary on the work, adds: “Gundlach et al. promise straightforward solution-based processes for manufacturing simple digital logic circuit applications without the need for more complex integrated circuit design, and/or the use of additional tedious and time-consuming fabrication steps.”

Further reading: “Contact-induced crystallinity for high-performance soluble acene-based transistors and circuits”, Gundlach et al., Nature Materials 7, 216 (2008).

Further reading: “Complexity made simple”, Natalie Stingelin-Stutzmann, Nature Materials 7, 171 (2008).

Figure: Optical micrographs of typical field-effect transistor structures show the effect of pre-treating contacts to promote organic crystal formation. The treated structure (left) shows crystal structure extending from the rectangular contacts and merging in the channel. The right-hand panel in contrast shows untreated contacts (source: NIST).

Article first published in Nanomaterials News.