Singlet Fission in Carotenoid Aggregates (SIFICA)

Singlet fission is the process whereby one photon creates two triplet excited states. If both triplet states could be harvested by a single-junction solar cell, the solar cell efficiency would increase by 1/3. There has been much academic and industrial interest in developing new materials for singlet fission, but to date no material has proved ideal.

Carotenoids are the most widespread of the natural pigments, important for photosynthesis, vision, human health and industry (market value $1.2bn). Surprisingly, carotenoids also appear to be excellent candidates for singlet fission sensitizers for solar cells: they have strong absorption, fast (<100fs) and loss-free singlet fission and they have the potential for energy-level tuning due the hundreds of naturally available molecules. However, problems remain: the triplet excitons are generally only short-lived in the solvent-based aggregates we have measured to date (90% decay in 1ns), making triplet harvesting difficult. A further problem is that a mechanism for triplet transfer to the solar cell has yet to be demonstrated. Here, we hope to solve these problems by using synthetic carotenoproteins designed to hold the carotenoid in a conformation which prevents triplet-triplet annihilation, allowing triplets to be long-lived. In addition, we propose to use the proteins to aid triplet harvesting through external spin-orbit coupling or energy transfer to a tethered nanoparticle.

We also propose to use these synthetic carotenoproteins as model systems to understand the fundamental energy landscape and dynamics in carotenoids and carotenoid dimers. Carotenoid dimers and aggregates are ubiquitous in nature, but their function is not yet understood. This is mainly due to the experimental and theoretical difficulty in studying them. Here we bring together experts in biochemistry, spectroscopy and theory to study model carotenoproteins with time-resolved spectrosopy and new theoretical models. This combination of resources and expertise provides us with the timely and exciting possibility of really understanding, controlling and exploiting carotenoid-based singlet fission for solar energy harvesting.

This is a 3-year project in a new field of interdisciplinary research. The short-term impacts are mainly enhancing the knowledge economy and training.

Scientific advances.
Despite their ubiquitousness and potential as singlet fission sensitizers, very little research has been done on the science of carotenoid dimers or aggregates. This project will begin to close the knowledge gap by creating new materials and applying new theoretical models. It could lead to a re-evaluation of the role of carotenoids in nature and as photonic materials. It may also open up new applications for non-natural proteins.

People.
The PDRAs employed on the grant will benefit from outstanding postdoctoral training: either from Sheffield's award-winning 'Think Ahead Programme' or Oxford's MPLS postdoc portal, from the opportunity to learn cutting-edge interdisciplinary experimental and theoretical techniques (e.g. synthetic biology, ultrafast spectroscopy, DMRG) and they will gain supervision skills by mentoring, training and helping the PhD students associated with this project.

This project will have an impact on the PI's career as it is her first large grant as PI. She will be supported by very experienced researchers (Prof. Hunter, Prof. Barford) and a lab manager who has 7 years' experience in managing large grants. As such, it is the ideal project to develop as a lead researcher.

Economy.
On the longer-term, it is difficult to predict impact. Should we be able to produce protein films which show efficient NIR emission from pairs of triplet excitons, we will collaborate with The University of Cambridge and Eight19 to develop and commercialise the technology. This commercialisation is likely to be important and will allow the UK to capitalise on its early lead in singlet fission research. It is also possible that other aspects of the research are picked up by companies either interested in water-soluble carotenoids or carotenoids with tunable electronic and optical properties (e.g. BASF, Boots, Nutrilite, Unilever, IBR) or any technological developments we make (Ossila).

Society.
In the long-term, any commercialisation or development of more efficient solar cells will benefit the global society by providing a clean energy source. Beyond a certain value, increases in solar cell efficiency can reduce cost making solar energy more affordable and competitive. Using biosynthetic, biodegradable carotenoids in cosmetics could help build the circular economy and boost the carotenoid market. In addition, as carotenoids are biosynthesised in algae and cyanobacteria (which can be used to clean harsh industrial wastewater), there is future scope for using these applications to add value to biofuel synthesis.