October 26, 2015

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

(October 26, 2015)  The mechanism behind a process known as singlet fission, which could drive the development of highly efficient solar cells, has been directly observed by researchers for the first time.

An international team of scientists have observed how a mysterious quantum phenomenon in organic molecules takes place in real time, which could aid in the development of highly efficient solar cells.

The researchers, led by the University of Cambridge, used ultrafast laser pulses to observe how a single particle of light, or photon, can be converted into two energetically excited particles, known as spin-triplet excitons, through a process called singlet fission. If 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 one photon is absorbed it leads to the formation of one 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 two spin-triplet excitons.

Working with researchers from the Netherlands, Germany and Sweden, the Cambridge team confirmed that this ‘two-for-one’ transformation involves an elusive intermediate state in which the two triplet excitons are ‘entangled’, a feature of quantum theory that causes the properties of each exciton to be intrinsically linked to that of its partner.

By shining ultrafast laser pulses – just a few quadrillionths of a second – on a sample of pentacene, an organic material which undergoes singlet fission, 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 through quantum dynamics. The results are reported today (26 October) in the journal Nature Chemistry.

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