Researchers at NIMS, Japan's National Institute for Materials Science, in Tsukuba, have successfully synthesized pure organic compounds without doping that display metallic conduction at ambient pressure. These stable, durable and easy to process materials could take organic electronics another step forward in next-generation solar cell electrodes and touch panels. [Kobayashi et al., Nature Mater (2016) doi:10.1038/nmat4768]
Conventional organic compounds lack the heavy elements that allow metals to carry an electric current. And, while Henry Letheby demonstrated conduction in polyaniline in 1862, it was not until the 1950s and charge-transfer complexes and the 1960s with derivatives of tetraiodopyrrole that the field was given a push. Ching W. Tang and Steven Van Slyke at Eastman Kodak demonstrated the first organic diode in 1987, while the potential for organic LEDs as the "phosphors" in a new type of display was revealed by Cambridge University's Bradley, Burroughes, and Friend. And, of course Alan J. Heeger, Alan G. MacDiarmid, and Hideki Shirakawa shared the 2000 Nobel Prize for Chemistry for their work on conductive polymers.
Now is the hour for single component organic conductive materials. The earliest work required pressures of at least 1 gigapascal (GPa) to make organic metallic and it was assumed all along that high pressures would be critical to conduction. The NIMS team has overturned this deceived wisdom with new molecular materials in which charge-carrying holes spontaneously arise and conduct electricity over a wide range of temperatures but critically at ambient pressure. Electrical conductivity of a film of zwitterionic tetrathiafulvalene (TTF)-extended dicarboxylate radical (TED) was 530 Siemens per centimeter at room temperature, but was almost double that at 50 Kelvin, 1,000 S/cm. The team's molecular orbital calculations on the material suggest that it has a prominent spin density gradient that has not been seen in other radical molecules, which may correlate with the mechanism by which single-component molecules exhibit metallic conduction.
The discovery of the metallic nature of TED films cold lead to the design of new highly conductive organic components. Importantly, the materials require no addition of a dopant to be conductive. Moreover, the ability to process or even print these materials makes them rather more practical than some other conductive organic materials, so fabricating devices could use well-known lithographic methods adapted for this new medium.
"Our next target is towards both academic and industrial directions, namely, mechanism of the metallization and application to printable electronics," Kobayashi told Materials Today.
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".