Scientists unlock a 100-year-old quantum secret to supercharge solar power

The study centers on a spin-radical organic semiconductor known as P3TTM. At the core of each molecule lies one unpaired electron, which gives it distinctive magnetic and electronic behavior. The work is the result of collaboration between Professor Hugo Bronstein's synthetic chemistry group in the Yusuf Hamied Department of Chemistry and Professor Sir Richard Friend's semiconductor physics team in the Department of Physics. These researchers previously designed this family of molecules for their bright luminescence, useful in organic LEDs, but the new paper in Nature Materials reveals something unexpected: when the molecules are packed closely together, their unpaired electrons interact much like those in a Mott-Hubbard insulator.

"This is the real magic," explained Biwen Li, the lead researcher at the Cavendish Laboratory. "In most organic materials, electrons are paired up and don't interact with their neighbors. But in our system, when the molecules pack together the interaction between the unpaired electrons on neighboring sites encourages them to align themselves alternately up and down, a hallmark of Mott-Hubbard behavior. Upon absorbing light one of these electrons hops onto its nearest neighbor creating positive and negative charges which can be extracted to give a photocurrent (electricity)."

To test this effect, the team built a solar cell using a thin film of P3TTM. When exposed to light, the device achieved nearly perfect charge collection efficiency, meaning almost every incoming photon was turned into usable electric current. Traditional organic solar cells require two materials -- one to donate electrons and another to accept them -- and this interface limits efficiency. In contrast, these new molecules perform the entire conversion process within a single substance. After a photon is absorbed, an electron naturally moves to a neighboring molecule of the same type, creating charge separation. The small amount of energy needed for this process, known as the "Hubbard U," represents the electrostatic cost of placing two electrons on the same negatively charged molecule.

Dr. Petri Murto in the Yusuf Hamied Department of Chemistry developed molecular structures that allow tuning of the molecule-to-molecule contact and the energy balance governed by Mott-Hubbard physics needed to achieve charge separation. This breakthrough means that it might be possible to fabricate solar cells from a single, low-cost lightweight material.

The discovery carries profound historical significance. The paper's senior author, Professor Sir Richard Friend, interacted with Sir Nevill Mott early in his career. This finding emerges in the same year as the 120th anniversary of Mott's birth, paying a fitting tribute to the legendary physicist whose work on electron interactions in disordered systems laid the groundwork for modern condensed matter physics.

"It feels like coming full circle," said Prof. Friend. "Mott's insights were foundational for my own career and for our understanding of semiconductors. To now see these profound quantum mechanical rules manifesting in a completely new class of organic materials, and to harness them for light harvesting, is truly special."

"We are not just improving old designs" said Prof. Bronstein. "We are writing a new chapter in the textbook, showing that organic materials are able to generate charges all by themselves."