̽��ֱ�� of Cambridge - Alex Chin

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

sc604 — Mon, 26 Oct 2015 16:15:13 +0000

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.

̽��ֱ��researchers, led by the ̽��ֱ�� 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. ̽��ֱ��results are reported today (26 October) in the journal Nature Chemistry.

“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 from the ̽��ֱ��’s Cavendish Laboratory, one of the study’s co-authors. “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.”

̽��ֱ��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. ̽��ֱ��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.

Using this approach, the team 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.

̽��ֱ��authors then developed a model which showed that when the molecules are vibrating, they possess new quantum states that simultaneously have the properties of both the light-absorbing singlet exciton and the dark triplet pairs. These quantum ‘super positions’, which are the basis of Schrödinger’s famous thought experiment in which a cat is – according to quantum theory – in a state of being both alive and dead at the same time, 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 ̽��ֱ��’s Cavendish Laboratory. “It is a crucial step towards opening up new routes to highly efficiency solar cells.”

̽��ֱ��research was supported by the European LaserLab Consortium, Royal Society, and the Netherlands Organization for Scientific Research. ̽��ֱ��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

̽��ֱ��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.

Harnessing the process of singlet fission into new solar cell technologies could allow tremendous increases in energy conversion efficiencies in solar cells

Alex Chin

Lawrence W Chin, David Turban and Alex W Chin

Pentacene molecules convert a single photon into two molecular excitations via the quantum mechanics of singlet fission

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Quantum waves at the heart of organic solar cells

fpjl2 — Thu, 12 Dec 2013 19:01:00 +0000

By using an ultrafast camera, scientists say they have observed the very first instants following the absorption of light into artificial yet organic nanostructures and found that charges not only formed rapidly but also separated very quickly over long distances - phenomena that occur due to the wavelike nature of electrons which are governed by fundamental laws of quantum mechanics.

This result surprised scientists as such phenomena were believed to be limited to "perfect" - and expensive - inorganic structures; rather than the soft, flexible organic material believed by many to be the key to cheap, 'roll-to-roll' solar cells that could be printed at room temperatures - a very different world from the traditional but costly processing of current silicon technologies.

̽��ֱ��study, published today in the journal Science, sheds new light on the mystery mechanism that allows positive and negative charges to be separated efficiently - a critical question that continues to puzzle scientists - and takes researchers a step closer to effectively mimicking the highly efficient ability to harvest sunlight and convert into energy, namely photosynthesis, which the natural world evolved over the course of millennia.

"This is a very surprising result. Such quantum phenomena are usually confined to perfect crystals of inorganic semiconductors, and one does not expect to see such effects in organic molecules - which are very disordered and tend to resemble a plate of cooked spaghetti rather than a crystal," said Dr Simon Gélinas, from Cambridge's Cavendish Laboratory, who led the research with colleagues from Cambridge as well as the ̽��ֱ�� of California in Santa Barbara.

During the first few femtoseconds (one millionth of one billionth of a second) each charge spreads itself over multiple molecules rather than being localised to a single one. This phenomenon, known as spatial coherence, allows a charge to travel very quickly over several nanometres and escape from its oppositely charged partner - an initial step which seems to be the key to generating long-lived charges, say the researchers. This can then be used to generate electricity or for chemical reactions.

By carefully engineering the way molecules pack together, the team found that it was possible to tune the spatial coherence and to amplify - or reduce - this long-range separation. "Perhaps most importantly the results suggest that because the process is so fast it is also energy efficient, which could result in more energy out of the solar cell," said Dr Akshay Rao, a co-author on the study from the Cavendish Laboratory.

Dr Alex Chin, who led the theoretical part of the project, added that, if you look beyond the implications of the study for organic solar cells, this is a clear demonstration of "how fundamental quantum-mechanical processes, such as coherence, play a crucial role in disordered organic and biological systems and can be harnessed in new quantum technologies".

̽��ֱ��work at Cambridge forms part of a broader initiative to harness high tech knowledge in the physics sciences to tackle global challenges such as climate change and renewable energy. This initiative is backed by both the UK Engineering and Physical Sciences Research Council (EPSRC) and the Cambridge Winton Programme for the Physics of Sustainability. ̽��ֱ��work at the ̽��ֱ�� of California in Santa Barbara was supported by the Center for Energy Efficient Materials, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award #DC0001009.

Researchers have been able to tune ‘coherence’ in organic nanostructures due to the surprise discovery of wavelike electrons in organic materials, revealing the key to generating “long-lived charges” in organic solar cells - material that could revolutionise solar energy.

One does not expect to see such effects in organic molecules - which [...] tend to resemble a plate of cooked spaghetti

Simon Gélinas

Simon Gelinas

This is the experimental setup used to generate femtosecond laser pulses which serve as an ultrafast "flash " for the camera so that very rapid phenomenon can be filmed

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