Just add water – Chemists discover a mechanism behind the doping of organic semiconductors

Semiconductors and their mastery have allowed us to develop the technology that underpins our modern society. These devices are responsible for a wide range of electronics, including printed circuit boards, computer chips and sensors.

The electrical conductance of semiconductors is between that of insulators, such as rubber, and that of conductors, such as copper. By doping materials with different impurities, scientists can control the electrical properties of a semiconductor. This is what makes them so useful in electronics.

Scientists and engineers have explored new types of semiconductors with attractive properties that could lead to breakthrough innovations. One class of these new materials is organic semiconductors (OSCs), which are based on carbon rather than silicon. CSOs are lighter and more flexible than their conventional counterparts, properties that lend themselves to all kinds of potential applications, such as flexible electronics, for example.

In 2014, Thuc-Quyen Nguyen, a professor at UC Santa Barbara, and her lab first reported on doping CVS with the help of Lewis acids in order to To increase the conductance of certain semiconductor polymers; However, nobody knew why this increase had occurred until now.

Through a collaborative effort, Nguyen and his collages have analyzed this mechanism and their unexpected discovery promises to give us greater control over these materials. The work was supported by the Ministry of Energy and the results appear in the newspaper Nature's materials.

Researchers at UC Santa Barbara collaborated with an international team from the University of Kentucky, the Humboldt University of Berlin and the Donghua University of Shanghai. "The doping mechanism using Lewis acids is unique and complex, so it requires a team effort," said Nguyen.

"This is the purpose of this article," said lead author Brett Yurash, PhD candidate at Nguyen's lab, "to understand why adding this chemical to the organic semiconductor increases its conductivity."

"People thought it was just Lewis acid acting on the organic semiconductor," he explained. "But it turns out that you only get that effect if there is water present."

Apparently, water plays a key role in this process. The Lewis Acid grabs a hydrogen atom in the water and transmits it to the OSC. The additional positive charge makes the OSC molecule unstable, so that an electron from a neighboring molecule migrates to cancel the charge. This leaves a positively charged "hole" which then contributes to the conductivity of the material.

"The fact that water plays any role was really unexpected," said Yurash, the newspaper's chief author.

Most of these reactions are performed in controlled environments. For example, experiments at UC Santa Barbara were conducted under dry conditions, under nitrogen atmosphere. There was no humidity in the room at all. However, it is clear that moisture had penetrated the box with the other materials. "A tiny amount of water is enough to have this doping effect," Yurash said.

Scientists, engineers and technicians must be able to controllably boost a semiconductor for convenience. "We have totally mastered silicon," he said. "We can do without the exact amount we want and it's very stable." On the other hand, boosting CSOs in a controllable way has been a huge challenge.

Lewis acids are actually quite stable dopants, and the team's findings apply quite widely, beyond the few CSOs and acids tested. Most OSC doping work has used molecular dopants that do not dissolve easily in many solvents. "Lewis acids, on the other hand, are soluble in common organic solvents, inexpensive and available in various structures," said Nguyen.

Understanding the mechanism at work should enable researchers to design even better dopants in a targeted way. "We hope this will be the springboard from which other ideas can be launched," Yurash said. In the end, the team hopes that this information will help push organic semiconductors toward a broader commercial realization.