A 'radical' molecule for solar energy harvesting
Scientists at the University of Cambridge have revealed a powerful new mechanism for harvesting light and turning it into electricity using a glowing organic semiconductor molecule.
The work arises from a collaboration between the synthetic chemistry team of Hugo Bronstein and the semiconductor physics team led by Richard Friend in the department of physics. They claim their findings, published in Nature Materials, could redefine the future of solar energy and electronics, and lead to lighter, cheaper, and simpler solar panels made from a single material.
Their paper 'Intrinsic intermolecular photoinduced charge separation in organic radical semiconductor' focuses on a spin-radical molecule called P3TTM.
At its centre sits a single, unpaired electron, giving it unique magnetic and electronic properties. This class of molecules has been developed to give efficient luminescence, as exploited in organic LEDs, but the new study reveals a hidden talent: when brought into close contact, their unpaired electrons interact in a manner strikingly similar to what is known as a Mott-Hubbard insulator.
The team demonstrated this by creating a solar cell from a P3TTM film. When light hit the device, it achieved a close-to-unity charge collection efficiency, meaning almost every photon of light was converted into a usable electrical charge.
In conventional molecular semiconductor solar cells, the conversion of an absorbed photon into electrons and holes (electricity) can only happen at the interface between two different materials where one acts an electron donor and the other as an electron acceptor, and this compromises overall efficiency.
In contrast, for these new materials, after a photon is absorbed, it is energetically “downhill” to move an electron from one molecule to an identical neighbouring molecule, thus creating electrical charges.
The energy required for this, often termed the “Hubbard U” is the electrostatic charging energy for double electron occupancy of the molecule that has become negatively charged.
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.
“It feels like coming full circle,” said Friend, who met Nevill Mott early in his career. “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 Bronstein. “We are writing a new chapter in the textbook, showing that organic materials are able to generate charges all by themselves”.
Reference
Li, B., Murto, P., Chowdhury, R. et al. Nat. Mater. (2025)
