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Entanglement at heart of 'two-for-one' fission in next-generation solar cells

last modified Nov 04, 2015 02:20 PM

An international team of scientists that include Winton Advanced Research Fellows Akshay Rao and Alex Chin and Winton Scholar Sarah Morgan, 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 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.

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. The results are reported in the journal Nature Chemistry.

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 exciton, known as a 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 take place.

To get around this, the team employed a powerful technique known as two-dimensional 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.

“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. “It is a crucial step towards opening up new routes to highly efficiency solar cells.”

The research was supported by the European LaserLab Consortium, Royal Society, and the Netherlands Organization for Scientific Research. The work at Cambridge forms part of a broader initiative to harness high tech knowledge in the physical sciences to tackle global challenges such as climate change and renewable energy. This initiative is backed by the UK Engineering and Physical Sciences Research Council (EPSRC) and the Winton Programme for the Physics of Sustainability.

Reference:Bakulin, Artem et. al. ‘Real-time observation of multiexcitonic states in ultrafast singlet fission using coherent 2D electronic spectroscopy.’ Nature Chemistry (2015). DOI: 10.1038/nchem.2371

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