Light and air boost conductivity of polymer semiconductors

Doping semiconductors such as silicon with carefully chosen elements that modify their properties forms the basis of all electronics. Researchers have now used air and light to dope organic semiconductors (Nature 2024, DOI: 10.1038/s41586-024-07400-5). Their simple, room-temperature doping method is a step toward high-performance, low-cost organic electronics.

Electronic devices based on organic semiconductors promise to be lightweight, flexible, and easy to make. The materials typically have low conductivity, though, so doping is critical for increasing performance. While silicon is doped with parts-per-million quantities of chemicals, organic materials require much larger fractions, making the doped organics “more like a composite,” says Simone Fabiano, a chemist at Linköping University. And that composition comes with drawbacks. Unreacted dopants can merge to form nonconductive islands, worsening the material’s electronic and structural properties.

Fabiano and his colleagues thought of doping with a light-activated catalyst instead of chemical dopants. Photocatalysts are used in organic synthesis for reduction-oxidation reactions, in which the catalysts shuttle electrons from one molecule to another. “And doping is essentially an oxidation or reduction process,” he says.

The researchers chose commercially available acridinium-based photocatalysts. Their dopant choice depended on whether they were creating an n-type semiconductor, which is flush with electrons for conductivity, or a p-type semiconductor, which is loaded with positive charges.

For p-doping, the team dipped various polymer semiconductor films into a solution of the photocatalyst and shined blue light on it in the presence of air. The catalyst absorbed energy from the light and shuttled electrons from the polymers to oxygen in the air, creating positive charges in the polymer. The polymers’ conductivities jumped from under 0.0001 siemens/cm to hundreds of siemens per centimeter after 10 min of light exposure. One polymer reached 3,000 S/cm.

When the researchers added triethylamine as a dopant to the solution and shined ultraviolet light in the presence of pure nitrogen, the photocatalyst transferred electrons from the dopant to the polymer. The resulting n-doped polymer’s conductivity increased by more than five orders of magnitude after 2 min of irradiation. It was even possible to simultaneously create both n-doped and p-doped semiconductors, the team found. For that, the researchers immersed films made of two different semiconductors in a photocatalyst solution and exposed it to light. The catalyst shuttled electrons from one film to the other. Fabiano says the researchers now plan to dig deeper into the mechanism underlying the doping and see whether they can extend it to other organic semiconductors.

“This new approach of electrical doping involving photoexcitation and the use of photocatalysts is creative and innovative,” says Bernard J. Kippelen, an organic optoelectronics expert at the Georgia Institute of Technology. The stability of the doping will need more detailed investigation, as will the practicality and scalability of the process, but he says that “for now, this paper reports a great new discovery.”