Researchers at the University of Cambridge have discovered a quantum phenomenon in an organic semiconductor, enabling highly efficient light-to-electricity conversion. This breakthrough, involving Mott-Hubbard behavior in the molecule P3TTM, could lead to simpler, cheaper solar panels made from a single material. The finding connects to foundational physics from a century ago and marks the 120th anniversary of physicist Sir Nevill Mott's birth.
In a study published in Nature Materials in 2025, scientists led by Professor Hugo Bronstein from the Yusuf Hamied Department of Chemistry and Professor Sir Richard Friend from the Department of Physics at the University of Cambridge revealed an unexpected quantum effect in the spin-radical organic semiconductor P3TTM. This molecule features an unpaired electron, giving it unique magnetic and electronic properties. When molecules pack closely, their unpaired electrons interact similarly to those in a Mott-Hubbard insulator, a behavior previously observed only in inorganic metal oxides.
"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)."
The team constructed a solar cell using a thin film of P3TTM, which achieved nearly perfect charge collection efficiency—converting almost every photon into usable current. Unlike traditional organic solar cells that require two materials for electron donation and acceptance, P3TTM handles the entire process internally. After light absorption, an electron moves to a neighboring molecule, creating charge separation with minimal energy cost, known as the 'Hubbard U.'
Dr. Petri Murto in the Yusuf Hamied Department of Chemistry designed molecular structures to optimize molecule-to-molecule contact and the energy balance required for this charge separation. This innovation suggests the potential for fabricating lightweight, low-cost solar cells from a single material.
The discovery holds historical resonance, emerging in the year of the 120th anniversary of Sir Nevill Mott's birth. Professor Friend, who knew Mott early in his career, reflected: "It feels like coming full circle. 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."
Professor Bronstein added: "We are not just improving old designs. We are writing a new chapter in the textbook, showing that organic materials are able to generate charges all by themselves."