How to harvest double the amount of current from solar cells

Researchers are trying to improve efficiency is to split energy available from visible photons into two, which leads to a doubling of the current in the solar cell.

Researchers in Cambridge and Mons have investigated the process in which the initial electronic excitation can split into a pair of half-energy excitations.

This 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.

The study shows that this 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 material 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 find 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.

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 fission and observe a key intermediate step never before seen.

The study has been published in the journal Nature Chemistry.