You may have learned in school that metals conduct electricity and plastics don’t, but for more than thirty years, scientists have been developing organic conductors and semiconductors that break those rules. Organic light-emitting diodes are now commonplace in LED screens, and conductive plastics could unlock a new wave of flexible electronic technologies, from electronic paper and bendable displays to printed circuitry using conductive ink.
However, even the best organic polymers are still far worse electrical conductors than their inorganic counterparts. “Typically, these polymers contain a backbone with alternating single and double bonds, making a path for electrons to hop between successive carbon atoms,” explained Dexter Tam, Scientist at A*STAR’s Institute of Materials Research and Engineering (IMRE). “But the polymer backbone can bend and twist, slowing down any electrical conduction—just like a car would slow down on a road that is winding and bumpy.”
Tam was interested in chemical components that could address this problem, eventually settling on a property called proquinoidal character, exhibited by special rings of carbon and nitrogen atoms. “These structures can stabilize a string of alternating single and double bonds,” Tam said. “Thus, their presence can flatten out the backbone of an organic polymer, making it a better conductor.”
In collaboration with colleagues from A*STAR’s Institute of High Performance Computing and Nanyang Technological University, Singapore, Tam was able to synthesize conjugated polymers containing BBTa26, a candidate proquinoidal monomer. When he tested these new polymers, he found that he had a new semiconducting material, which was a thousand to a billion times more conductive than other organic semiconductors.
The team was able to improve the conductivity further by adding an impurity to supply more charge carriers, a common modification for both organic and inorganic semiconductors. “When we did this, our final products had conductivities reaching 100 S/cm, a thousand times better than other widely researched and used commercial conducting polymers,” Tam stated.
Looking forward, Tam hopes to further improve conjugated polymers using a variety of other proquinoidal monomers. “We are always interested in finding new material properties to incorporate into our conjugated polymers,” Tam said. “Our latest versions incorporate high-spin electronic states that could further improve performance and allow applications in spintronics.”
The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering (IMRE) and the Institute of High Performance Computing (IHPC).