Singlet fission in polyenes

Lead Research Organisation: University of Sheffield
Department Name: Physics and Astronomy

Abstract

In current solar cells much of the UV and blue light absorbed is lost as heat, as the energy of the absorbed photon is much higher than the bandgap of the semiconductor material, and therefore the collected electronic energy. One mechanism for harvesting all of the absorbed photon energy is to exploit the 'singlet exciton fission' process which occurs in some organic semiconductors. Singlet fission is a process whereby the primary excited state (singlet exciton) can split into two distinct triplet excitons which can both be harvested. In this way, one absorbed photon creates two collected charges, producing a solar cell with up to 200% quantum efficiency.

Singlet fission in polyenes (linear conjugated carbon chains) occurs when there is enough space for two triplet excitons to sit near each other: either on a very long polymer chain, or on two closely spaced neighbouring molecules. Polyenes demonstrate strong photoabsorption in UV-visible region, are solution-processable and demonstrate very efficient and robust singlet fission. They are therefore excellent candidates for singlet fission solar cells.

To exploit the singlet fission mechanism and the polyene class of materials, we need better understanding of how to describe singlet fission in polyenes and how to control it through material design. In addition, we urgently need to demonstrate whether the triplet excitons created through singlet fission can be efficiently ionised to create charges. In this project, we will answer these questions using a combination of cutting-edge time-resolved spectroscopic techniques, model samples and high-level theory.

Planned Impact

This is a 1-year project mostly dealing with fundamental physics. The short-term impacts are mainly enhancing the knowledge economy and training students/postdocs.

Scientific advances
There are gaps in the understanding of polyene (including carotenoid) excited state physics that this project will fill, for example the mechanism of singlet fission in polyenes and whether they can be used as singlet fission sensitisers for improving solar cell efficiency.

Techniques
During the project, we will complete the building of a state-of-the-art laser facility at the University of Sheffield. This will include experiments that, to the best of my knowledge, have not yet been performed with such good time resolution anywhere in the world (for example, broadband transient grating photoluminescence luminescence spectroscopy and transient absorption spectroscopy with mid-IR (2-4um) probe, both with sub-40fs resolution).

People
The PDRA employed on the grant if successful (Nick Paul) will be starting his first postdoc. He will gain supervision skills through training, mentoring and helping the PhD students. The students will benefit from the additional training provided by the PDRA and from the amazing opportunity to travel to international conferences to meet world-experts and to present their work.
This proposal will impact the PI's career significantly as this is not only her first grant, she has also just returned from two consecutive maternity leaves (two children in two years) and moved to a new institution. Following this career break, she must quickly build up her new research group starting with the postdoc named on the grant. This grant will provide the seed with which to apply for more funding to build a highly successful and productive research team.

Economy
On the longer term, it is difficult to predict the impact. It is likely to lead to new funding to research and develop singlet fission sensitised solar cells with better efficiency than today's best solar cells. This would impact the economy most likely in the form of new products in established companies such as Eight19, SolarWorld or ZNshine.

Publications

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Musser AJ (2019) Heavy-atom effects on intramolecular singlet fission in a conjugated polymer. in The Journal of chemical physics

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Musser AJ (2019) Triplet-Pair States in Organic Semiconductors. in Annual review of physical chemistry

 
Description We now know much more about singlet exciton fission in carotenoids than we did at the start of this short project. For example, singlet fission does not depend on aggregate or molecular structure. We now understand how the process of singlet fission depends on the length of the conjugated polymer chain and how to describe the excited-state absorption spectra of these materials. Our previous research sparked a small controversy in the literature which we resolve with this work.
Exploitation Route Previous research sparked a small controvery, which this research resolves. This helps researchers in the singlet fission and carotenoid fields to better understand and exploit singlet exciton fission.
Sectors Agriculture, Food and Drink,Chemicals,Energy,Pharmaceuticals and Medical Biotechnology

 
Description They have been very useful in public engagement activities as they are intuitive.
First Year Of Impact 2017
Sector Other
Impact Types Societal

 
Description Singlet Fission in Carotenoid Aggregates (SIFICA)
Amount £294,636 (GBP)
Funding ID EP/S002766/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 01/2019 
End 12/2021
 
Description Singlet Fission in Carotenoid Aggregates (SIFICA)
Amount £851,312 (GBP)
Funding ID EP/S002103/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 01/2019 
End 12/2021
 
Description Biology partnership 
Organisation University of Sheffield
Country United Kingdom 
Sector Academic/University 
PI Contribution Based on the findings from this project, we began a collaboration with biologists (Hunter FRS group) to examine the biological effects of singlet fission in polyenes (eg carotenoids). We measured the samples they produced and worked together to understand the findings.
Collaborator Contribution 1 PhD student made synthetic proteins, another extracted biological carotenoid complexes. We measured their samples and together understood the findings.
Impact Grant has been written and was accepted. Papers are being prepared.
Start Year 2017
 
Description Chemistry partnership 
Organisation Imperial College London
Country United Kingdom 
Sector Academic/University 
PI Contribution We asked Prof. Martin Heeney for specific materials for singlet fission. They synthesised them, we measured them and analysed the results.
Collaborator Contribution PhD student at Imperial synthesised and performed initial characterisation on a range of polyenes for singlet fission studies. Sent them to Sheffield for ultrafast characterisation by us.
Impact Paper under preparation.
Start Year 2016
 
Description Theory partnership 
Organisation University of Oxford
Country United Kingdom 
Sector Academic/University 
PI Contribution We performed experiments, Prof. Barford (Oxford Chemistry) used the basis of these measurements to begin a research effort (1 PhD student) into calculations on understanding singlet fission in polyenes.
Collaborator Contribution 1 PhD student + supervisor time for calculations.
Impact 3-year grant
Start Year 2017
 
Description Festival of the Mind exhibition 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact Collaboration with 4 artists to describe the research in different ways. From this project, a picture entitled 'exciton' and a lobster-flying fairy sculpture. The posters are now up in the department, where they are discussed at open days, and mugs were produced and passed out to various audiences.
Year(s) Of Engagement Activity 2017