Recyclable printed electronics without using hazardous chemicals

Researchers at Duke University have developed the first fully recyclable printed electronics using water instead of toxic chemicals in the fabrication process, in a breakthrough that could lead to a more environmentally friendly electronics industry. With the amount of discarded electronics ever-growing, their results show that a relatively complex device, here a thin-film transistor, can be completely printed in a process involving nothing but water.

One of the main difficulties in electronics manufacturing is to ensure many layers of components are safely secured on top of each other to produce complex devices, especially for printed electronics. To obtain a water-based ink where the carbon nanotubes do not clump together and spread evenly on a surface, a detergent-like surfactant is usually introduced. However, ink produced in this way does not provide a layer of carbon nanotubes dense enough for a high current of electrons to travel across.

The team had previously shown the first fully recyclable printed electronics with devices that employed three carbon-based inks: semiconducting carbon nanotubes, conductive graphene and insulating nanocellulose. As reported in Nano Letters [Lu et al. Nano Lett. (2023) DOI: 10.1021/acs.nanolett.2c04196], to adapt this process for just water, the nanotubes were the main challenge. Using chemicals involves printing multiple passes of the nanotubes until there are sufficient, but water doesn’t increase in density in this way due to the surfactant that keeps the carbon nanotubes from clumping also stopping additional layers from sticking to the first.

In standard manufacturing, these surfactants would be removed with either very high temperatures, requiring lots of energy or harsh chemicals. A cyclical process was therefore developed where the device is rinsed in water before being dried in a relatively low heat before being printed on again. As the amount of surfactant used in the ink is tuned down, the inks and processes can produce create fully functional, recyclable and water-based transistors.

Although the performance of their thin-film transistors is not yet on a par with the best currently in the market, they’re sufficiently viable to demonstrate the potential of such environmentally friendly processes. The method could also be used in the production of other electronic components such as screens and displays where the backplane electronics are primarily made of thin-film transistors, and the team now hope to improve the performance of their transistors.

As study leader Aaron Franklin told Materials Today, “Now that we have shown this is possible, it opens the way for ongoing research into water-only approaches that improve performance, reliability, and application readiness.”

“Now that we have shown this is possible, it opens the way for ongoing research into water-only approaches that improve performance, reliability, and application readiness.”Aaron Franklin