Electrically conductive plastics for batteries, solar cells

An emerging class of electrically conductive plastics called ‘radical polymers’ may bring low-cost, transparent solar cells, flexible and lightweight batteries, and ultrathin antistatic coatings for consumer electronics and aircraft.

Researchers at Purdue University have established the solid-state electrical properties of one such polymer, called PTMA, which is about 10 times more electrically conductive than common semiconducting polymers.

“It’s a polymer glass that conducts charge, which seems like a contradiction because glasses are usually insulators,” said Bryan Boudouris, an assistant professor of chemical engineering at Purdue University.

The polymer is easy to manufacture, resembling Plexiglas, an inexpensive transparent plastic found in numerous products. However, unlike Plexiglas it conducts electricity. The PTMA is in a class of electrically active polymers that could bring inexpensive transparent solar cells; antistatic and antiglare coatings for mobile phone displays; antistatic coverings for aircraft to protect against lightning strikes; flexible flash drives; and thermoelectric devices, which generate electricity from heat.

The polymers have seen commercial use in new types of batteries. However, finding widespread practical applications for the polymers will require increasing the conductivity another 100 to 1000 times, Boudouris said.

Polymers are strings of molecules with a central backbone and may contain side chains called ‘pendant groups’ that dangle from the central structure. In radical polymers, it’s these pendant groups that allow charge to be transported, conducting current.

To create the radical polymer, the researchers used a procedure called deprotection, which involves replacing a specific hydrogen atom in the pendant group with an oxygen atom, converting it into a so-called radical group. “We just finally studied deprotection in a way others had not to learn how it affects the electronic properties of the radical polymers,” Boudouris said. Electrons surround an atom’s nucleus in ‘shells’, and these electrons are usually paired. The oxygen atom in PTMA, however, has one unpaired electron in its outer shell, making it amendable to transporting charge.

“You have to control the deprotection process very well because it makes the conductivity vary by orders of magnitude,” he said. The researchers have determined that the deprotection step can lead to four distinct chemical functionalities of the radical polymer, two of which are promising for increasing the conductivity of the polymer.

“So manipulating the reaction conditions for this deprotection step, and monitoring closely the resultant chemical functionalities, is critical in tuning the electrical properties of radical polymers,” Boudouris said.