Organic semiconductors need to stay hydrated during doping

Semiconductors – and our mastery of them – have been instrumental in the development of the technology that underpins our modern society. These devices are responsible for a wide range of electronics, including circuit boards, computer chips and sensors.

The electrical conductance of semiconductors falls between those of insulators like rubber and conductors like copper. By doping a semiconductor with different impurities, scientists can control its electrical properties. This is what makes them so useful in electronics.

Scientists and engineers have been exploring new types of semiconductors with attractive properties that could result in revolutionary innovations. One example is organic semiconductors (OSCs), which are based on carbon rather than silicon. OSCs are lighter and more flexible than their conventional silicon counterparts, lending themselves to all sorts of potential applications, such as flexible electronics, for instance.

In 2014, Thuc-Quyen Nguyen at the University of California (UC), Santa Barbara and her lab first reported that doping OSCs with Lewis acids can increase the conductance of some semiconducting polymers. But no one knew why this happened – until now.

Through a collaborative effort, Nguyen and her colleagues have uncovered the mechanism, and their unexpected discovery promises greater control over semiconductors. The work was supported by the US Department of Energy and is reported in a paper in Nature Materials.

For this study, Nguyen and her team at UC Santa Barbara collaborated with colleagues from the University of Kentucky, Humboldt University of Berlin in Germany and Donghua University in Shanghai, China. "The doping mechanism using Lewis acids is unique and complex; therefore, it requires a team effort," Nguyen explained.

"That's what this paper is all about," said lead author Brett Yurash, a doctoral candidate in Nguyen's lab, "figuring out why adding this chemical to the organic semiconductor increases its conductivity."

"People thought it was just the Lewis acid acting on the organic semiconductor. But it turns out you don't get that effect unless water is present."

Apparently, water mediates a key part of this process. The Lewis acid grabs a hydrogen ion (H⁺) from the water and passes it over to the OSC. This extra positive charge makes the OSC molecule unstable, so an electron from a neighboring molecule migrates over to cancel out the charge. This leaves a positively charged ‘hole’ that then contributes to the material's conductivity.

"The fact that water was having any role at all was really unexpected," said Yurash, the paper's lead author.

These kinds of experiments are generally performed in controlled environments; for example, the experiments at UC Santa Barbara were conducted in dry conditions under a nitrogen atmosphere. There wasn't supposed to be any humidity in the chamber at all. But clearly some moisture did make it into the chamber with the other materials. "Just a tiny amount of water is all it took to have this doping effect," Yurash said.

Scientists, engineers and technicians need to be able to controllably dope a semiconductor in order for it to be practical. "We've totally mastered silicon," explained Yurash. "We can dope it the exact amount we want and it's very stable." In contrast, controllably doping OSCs has been a huge challenge.

Lewis acids are actually pretty stable dopants, and the team's findings apply fairly broadly, beyond simply the few OSCs and acids they tested. Most of the doping work on OSCs has used molecular dopants, which don't dissolve readily in many solvents. "Lewis acids, on the other hand, are soluble in common organic solvents, cheap and available in various structures," Nguyen said.

Understanding the mechanism at work should allow researchers to design even better dopants. "This is hopefully going to be the springboard from which more ideas launch," Yurash said. Ultimately, the team hopes these insights will help push organic semiconductors toward broader commercial realization.

This story is adapted from material from the University of California, Santa Barbara, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.