Spectroscopy Elucidates Solar Singlet Fission

Better understanding of singlet fission, an electrical process triggered by light in some materials, could enable solar cells twice as efficient as those used today.

An international team led by researchers at the University of Cambridge used ultrafast laser pulses to observe how single photons can be converted into two spin-triplet excitons in the organic material pentacene.

In conventional semiconductors such as silicon, photon absorption leads to the formation of free electrons that can be harvested as electrical current. Certain other materials undergo singlet fission instead, where two spin-triplet excitons are formed for every photon absorbed.

The researchers confirmed that this "two-for-one" transformation involves an elusive intermediate state in which the two triplet excitons are entangled, a feature of quantum mechanics that causes the properties of each exciton to be intrinsically linked to that of its partner.

Pentacene molecules convert a single photon into two molecular excitations via the quantum mechanics of singlet fission. Courtesy of Lawrence W. Chin, David Turban and Alex W. Chin.

By shining ultrafast laser pulses on a sample of pentacene, the researchers were able to directly observe this entangled state for the first time, and showed how molecular vibrations make it both detectable and drive its creation.

"Harnessing the process of singlet fission into new solar cell technologies could allow tremendous increases in energy conversion efficiencies in solar cells," said research fellow Alex Chin. "But before we can do that, we need to understand how exciton fission happens at the microscopic level. This is the basic requirement for controlling this fascinating process."

The key challenge for observing real-time singlet fission is that the entangled spin-triplet excitons are essentially dark to almost all optical probes, meaning they cannot be directly created or destroyed by light.

In materials like pentacene, the first stage — which can be seen — is the absorption of light that creates a single, high-energy, spin-singlet exciton. The subsequent fission of the singlet exciton into two less-energetic triplet excitons gives the process its name. But the ability to see what is going on vanishes as the process occurs.

To get around this, the team used ultrafast 2D electronic spectroscopy, which involves illuminating the material with a coordinated sequence of ultrashort laser pulses and then measuring the light emitted by the excited sample. By varying the time between the pulses in the sequence, it is possible to follow in real time how energy absorbed by previous pulses is transferred and transformed into different states.

Using this approach, the team found that when the pentacene molecules were vibrated by the laser pulses, certain changes in the molecular shapes caused the triplet pair to become briefly light-absorbing, and therefore detectable by later pulses. By carefully filtering out all but these frequencies, a weak but unmistakable signal from the triplet pair state became apparent.

When the molecules are vibrating, the researchers said, they possess new quantum states that simultaneously have the properties of both the light-absorbing singlet exciton and the dark triplet pairs. These quantum superpositions not only make the triplet pairs visible, they also allow fission to occur directly from the moment light is absorbed.

"This work shows that optimized fission in real materials requires us to consider more than just the static arrangements and energies of molecules; their motion and quantum dynamics are just as important," said research fellow Akshay Rao. "It is a crucial step towards opening up new routes to highly efficiency solar cells."

Funding came from the European LaserLab Consortium, Royal Society, Netherlands Organization for Scientific Research, U.K. Engineering and Physical Sciences Research Council, and Winton Program for the Physics of Sustainability.

The research was published in Nature Chemistry (doi: 10.1038/nchem.2371).