The researchers, led by University of Cambridge, used ultrafast laser pulses to observe how a photon, can be converted into 2 energetically excited particles, known as spin-triplet excitons, through a process called singlet fission. If the process of singlet fission can be controlled, it could enable solar cells to double the amount of electrical current that can be extracted.
In conventional semiconductors such as silicon, when 1 photon is absorbed it leads to the formation of 1 free electron that can be harvested as electrical current. However certain materials undergo singlet fission instead, where the absorption of a photon leads to the formation of 2 spin-triplet excitons. The team confirmed this ‘two-for-one’ transformation involves an elusive intermediate state in which the 2 triplet excitons are ‘entangled’.
By shining ultrafast laser pulses – just a few quadrillionths of a second – on a sample of pentacene, an organic material which undergoes singlet fission, they saw this entangled state for the 1st time, and showed how molecular vibrations make it both detectable and drive its creation through quantum dynamics.
“Harnessing the process of singlet fission into new solar cell technologies could allow tremendous increases in energy conversion efficiencies in solar cells,” said Dr 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. They used 2D spectroscopy, which involves hitting the material with a co-ordinated 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, they found that when the pentacene molecules were vibrated by the laser pulses, certain changes in the molecular shapes cause 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, they possess new quantum states that simultaneously have both light-absorbing singlet exciton and the dark triplet pairs. These quantum ‘super positions’, which are the basis of Schrödinger’s experiment not only make the triplet pairs visible, they also allow fission to occur directly from the moment light is absorbed.
“This work shows that optimised 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 Dr Akshay Rao, from the University’s Cavendish Laboratory. “It is a crucial step towards opening up new routes to highly efficiency solar cells.” http://www.cam.ac.uk/research/news/entanglement-at-heart-of-two-for-one-fission-in-next-generation-solar-cells
Pentacene molecules convert a single photon into two molecular excitations via the quantum mechanics of singlet fission. Credit: Lawrence W Chin, David Turban and Alex W Chin
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