New organic molecule set to transform solar energy harvesting

In a discovery that bridges a century of physics, scientists have observed a phenomenon, once thought to be the domain of inorganic metal oxides, thriving within a glowing organic semiconductor molecule. This breakthrough, led by the University of Cambridge, reveals a powerful new mechanism for harvesting light and turning it into electricity. This could redefine the future of solar energy and electronics, and lead to lighter, cheaper, and simpler solar panels made from a single material.

The research focuses on a spin-radical organic semiconductor molecule called P3TTM. At its centre sits a single, unpaired electron, giving it unique magnetic and electronic properties. This work arises from a collaboration between the synthetic chemistry team of Professor Hugo Bronstein in the Yusuf Hamied Department of Chemistry and the semiconductor physics team led by Professor Sir Richard Friend in the Department of Physics.  They have developed this class of molecules to give very efficient luminescence, as exploited in organic LEDs, but the new study, published in Nature Materials, reveals their hidden talent: when brought into close contact, their unpaired electrons interact in a manner strikingly similar to 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 neighbours. But in our system, when the molecules pack together the interaction between the unpaired electrons on neighbouring sites encourages them to align themselves alternately up and down, a hallmark of Mott-Hubbard behaviour. Upon absorbing light one of these electrons hops onto its nearest neighbour creating positive and negative charges which can be extracted to give a photocurrent (electricity).”

The team demonstrated this by creating a solar cell from a P3TTM film. When light hit the device, it achieved a remarkable 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.

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”.