Researchers at the Cambridge Cavendish Laboratory and Mons have investigated a process in which the initial photonic electronic excitation can split into a pair of half-energy excitations. The phenomenon can happen in certain organic molecules when the quantum mechanical effect of electron spin sets the initial spin ‘singlet’ state to be double the energy of the alternative spin ‘triplet’ arrangement.

When a photon is absorbed it creates a single electronic excitation that is then separated into an electron and a positively charged hole, irrespective of the light energy. One way to improve efficiency is to split energy available from visible photons into two, a kind of fission that leads to a doubling of the current in the solar cell.

Laser Apparatus to Study Singlet Fission at Cambridge. Image Credit: Sebastian Albert-Seifried.  Click image for the largest view.

Laser Apparatus to Study Singlet Fission at Cambridge. Image Credit: Sebastian Albert-Seifried. Click image for the largest view.

Solar cells offer the opportunity to harvest abundant, renewable energy. Although the highest energy light occurs in the ultraviolet and visible spectrum, most solar energy is in the infrared. There is a trade-off in harvesting this light, so that solar cells are efficient in the infrared but waste much of the energy available from the more energetic photons in the visible part of the spectrum.

The group’s study paper has been published in the journal Nature Chemistry where the team reports that the process of singlet fission to pairs of triplets depends very sensitively on the interactions between molecules. By studying this process when the molecules are in solution it is possible to control when this process is switched on.

When the active material in the solution is very dilute, the distance between molecules is large and singlet fission does not occur. When the solution is concentrated, collisions between molecules become more frequent. The researchers found that the fission process happens as soon as just two of these molecules are in contact, and remarkably, that singlet fission is then completely efficient – so that every photon produces two triplets.

This fundamental study provides new insights into the process of singlet fission and demonstrates that the use of singlet fission is a very promising route to improved solar cells. Chemists will be able to use the results to make new materials, said the team from Cambridge’s Cavendish Laboratory, who are currently working on ways to use these solutions in devices.

Dr. Brian Walker, a research fellow in the Cavendish Lab’s Optoelectronics group, who led the study explained saying, “We began by going back to fundamentals; looking at the solar energy challenge from a blue skies perspective. Singlet fission offers a route to boosting solar cell efficiency using low-cost materials. We are only beginning to understand how this process works, and as we learn more we expect improvements in the technology to follow.”

The team used a combination of laser experiments, which measure timings with extreme accuracy, with chemical methods used to study reaction mechanisms. This dual approach allowed the researchers to slow down the fission and observe a key intermediate step never before seen.

“Very few other groups in the world have laser apparatus as versatile as ours in Cambridge,” added Andrew Musser, a researcher who collaborated in the study. “This enabled us to get a step closer to working out exactly how singlet fission occurs.”

The news suggests a new breakthrough in solar energy harvesting, but may be a furthering of the electron spin research that may well be the basis for this new development.  So far the research is in the lab using lasers.  We’ll keep an eye out for a cell built to work in the sunlight.

The Brit’s idea could be a means to circumvent the Shockley–Queisser limit in single-junction solar cells.  The main problem for now is the conversion of the energy in the collector to an electrical output.  It will be interesting to see just how efficient the new cell can be in the sunshine.


Name (required)

Email (required)


Speak your mind