It’s not just grocery bags and disposable cutlery that are made of plastic. Most digital displays on our smartphones and smart televisions contain the seemingly omnipresent material, too. In these electronics, plastics take the form of conjugated polymers with unique chemical structures that give them specialized optical and electrical properties.
While such conjugated polymers may not conduct electricity as well as metals and inorganic semiconductors, other properties give them a distinct edge. Not only are they cost-effective and easier to produce at scale, these polymers also have more versatile and customizable structures and functions.
To make such polymers more electrically conductive, materials scientists use a process called doping, where impurities called dopants are introduced into the polymers to facilitate electron flow. In recent years, scientists have started exploring in-depth the relationship between the type of dopants used and the resulting conductivity of conjugated polymers. However, given the complicated nature of dopant synthesis, developing novel doping techniques has been slow.
Heeding the call for next-generation dopant technologies is Dexter Tam from A*STAR’s Institute of Materials Research and Engineering (IMRE). Together with his team, Tam is studying the feasibility of using a benzyl viologen radical cation (BVc+) as a dopant for various conjugated polymers.
According to him, BVc+ is an exceptionally robust dopant as it facilitates electron transport between the polymer fibers. “Our previous experience in organic electrochromics suggests that BVc+, which is easily made using commercial reagents, is a promising dopant,” added Tam.
Putting their hypothesis to the test, the team created a conducting plastic conjugated polymer called poly(benzimidazobenzophenanthrolinedione) (BBL) using BVc+ as a dopant. To achieve this, they used a method called sequential-solution doping.
“To achieve high conductivity, you should have delocalized electrons, which we were able to do by using a dopant and a doping technique that is less disruptive to polymer packing during the doping process,” explained Tam.
Further characterization of the doped BBL revealed that sequential-solution doping kept the polymer’s unique rigid, laddered structure—a feature that contributes to its high electron mobility and resulting conductivity. The BVc+ dopant also facilitated enhanced electron transfer between BBL’s fibers during the reaction, further elevating its thermal stability and ability to conduct electricity. When tested, BBL doped with BVc+ broke records set by other semiconductors, achieving thermal stability of up to 100°C over four days.
With its high thermoelectric performance, thermal stability, and relative ease of preparation, Tam believes that doping BBL with BVc+ opens the doors for new possibilities in the flexible electronics industry. Moving forward, the team plans to further their work on polymer thermoelectrics to develop more novel conjugated polymer materials.
The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering (IMRE).
