A new organic semiconductor molecule could lead to lighter and simpler solar panels made entirely from a single material.

For almost a century, physics said it couldn’t be done with a single, simple organic material. But thanks to the discovery by the University of Cambridge, this could be possible. 

The team observed a powerful light-harvesting mechanism, long thought limited to inorganic materials (metal oxides). This interesting mechanism thrived within a simple, glowing organic semiconductor molecule called P3TTM.  

Announced on October 1, Cambridge describes the discovery that “bridges a century of physics”. 

“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,” said Professor Hugo Bronstein, the study author, in the press release. 

The chemical structure of P3TTM next to a photo of luminescence from a film of P3TTM: a thin film emits red light from the radical doublet excited state.

Unpaired electron magic

The research centres on this unique molecule, a spin-radical organic semiconductor.

A single, unpaired electron sits at the molecule’s core, which gives unique electronic and magnetic properties.

According to lead researcher Biwen Li, the key to the discovery lies in how the molecules interact. 

When packed tightly, the unpaired electrons on neighbouring sites align alternately up and down. 

This interaction is a hallmark of what is known in condensed matter physics as Mott-Hubbard behaviour –  a phenomenon previously reserved for complex, inorganic metal oxides. 

The hopping action instantly creates positive and negative electrical charges (a current) that can be easily extracted.

“This is the real magic,” noted Li. “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).”

To demonstrate this concept, the team created a solar cell using a film of the P3TTM molecule.  

Hitting the device with light resulted in a fantastic conversion rate, achieving “close-to-unity charge collection efficiency”.

The conversion rate showcases that nearly all absorbed photons were turned into usable electricity. 

In comparison, the standard designs rely on an interface between two materials – one to donate electrons and another to accept them – to separate charges and generate electricity. 

Interestingly, the new organic material can generate charges all by itself. 

Tribute to a legend

The experts created molecular structures in this new work to control the molecule-to-molecule contact and maintain the Mott-Hubbard energy balance. 

This design provided the key to achieving charge separation and could pave the way for fabricating single-material, low-cost solar cells.

Furthermore, the development honours physicist Sir Nevill Mott, whose work on electron interactions laid the groundwork for modern condensed matter physics.

And now, decades later, his quantum mechanical rules are being harnessed to power the future. 

“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,” said Professor Sir Richard Friend, the paper’s senior author.  

By solving a century-old physics problem, the Cambridge collaboration has unlocked a powerful new path toward creating highly efficient, easily manufactured solar cells to accelerate the shift to sustainable energy. 

The findings were published in the journal Nature Materials.