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Winton Programme for the Physics of Sustainability

Department of Physics

Dr Brian Walker (Optoelectronics)


In pursuit of viable sustainable energy sources, there has been much progress due to advances in engineering and infrastructure. However, because existing technologies contain certain intrinsic limitations, there are also large potential gains through the discovery of new physics. My work is centered on the elucidation of the mechanism for singlet fission in polycrystalline organic materials. Singlet fission is the process by which a chromophore in a singlet excited state gives rise to a pair of triplets with opposite spin. By dissociating these triplets in a suitable device, we may be able to generate multiple charge carriers from a single photoexcitation. Such a process is important, because it would open the possibility that a low-cost photovoltaic cell could achieve power conversion efficiencies higher than the fundamental limits of simple silicon or thin film cells currently in production. I aim to study the materials properties that give rise to efficient fission using experimental techniques pioneered here in the Optoelectronics Group at the Cavendish Laboratory.

Project completed August 2013

The pump prime grant paid for a bespoke laboratory apparatus—a set of highly sensitive photodetectors— that has enabled new science since we ordered and installed them in 2012. Not only has there been progress toward the grant’s objectives in less than two years, there has also been an increase in cross-disciplinary collaborations within Cambridge and overseas. The impact of this grant is apparent from the outputs below. [1] Here are some highlights:

  • We demonstrated singlet fission in a single polymer chain, and we unexpectedly found that the process occurs through high-lying excited states. Such high-lying states are not normally productive, and this result may lead to improved technologies for harvesting light.
  • We discovered a new mechanism that that plastic solar cells can use to reduce losses and boost solar cell efficiencies.
  • With our colleagues in chemistry, we are investigating new devices that use nanoparticles with the supramolecular architecture of photosystem II—and our new experiments have uncovered new physics that is entirely unexplored.
  • We are studying new hybrid solar cells in ways that were completely inaccessible 18 months ago—and these solar cells real potential to exceed the physical limits on existing solar cell technologies.

 Currently, the apparatus is continuing to serve the needs of around 10 full-time users, in addition to guests of the laboratory. It operates nearly 24/7.

[1] Note that the Pump-Prime grant did not pay for the conference attendance or publication fees associated with these outputs. Rather, the grant made all of this work possible.


Winton Annual Report 2019

Latest news

Manipulation of Quantum Entangled Triplet Pairs

7 January 2021

Researchers have uncovered a new technique to create and manipulate pairs of particle-like excitations in organic semiconductors that carry non-classical spin information across space, much like the entangled photon pairs in the famous Einstein-Podolsky-Roden “paradox”.

Machine learning algorithm helps in the search for new drugs

20 March 2019

Researchers have designed a machine learning algorithm for drug discovery which has been shown to be twice as efficient as the industry standard.