High Energy Proton
Infrared Emission
Singlet
Triplet
New materials to boost solar cells Conventional solar cells do not effectively utilise all of the sunlight which reaches them, which limits their efficiency. An effective singlet fission photon multiplication film would allow solar cells to harvest energy from a greater proportion of the solar spectrum and so improve efficiency, as Dr Victor Gray explains. The earth receives an enormous amount of energy from the sun every day, yet only a relatively small proportion of it is converted into electricity. Currently based at the University of Cambridge in the UK, Dr Victor Gray is conducting fundamental research into a process called singlet fission, which could be utilised to enhance the efficiency of solar cells. “I aim to understand singlet fission in certain organic molecules, so we can then develop materials that can eventually be used in solar cells,” he outlines. In singlet fission, a high-energy singlet exciton is converted into two triplet excitons (electrons are unpaired), each with around half the energy of the singlet exciton. “A singlet is a molecule with all electrons paired, typically pictured as two antiparallel arrows, whereas a triplet is a molecules with unpaired electrons, typically pictured as two parallel arrows,” says Dr Gray. The energy of these triplets with unpaired electrons is then closer to the band gap of the semiconductor material, potentially offering a route to harvesting energy from more of the solar 38
spectrum and improving efficiency. However, challenges still remain in terms of applying these materials in solar cells. “There is a lot of interest in using singlet fission materials in solar cells, but they’re difficult to use because the triplets that are formed are non-emissive and hard to extract,” explains Dr Gray.
Photon multiplication film This is an issue Dr Gray aims to overcome by using emissive quantum dots in the singlet fission photon multiplication film that he and his colleagues are developing, an optical layer which can be added to a solar module. The absorbing singlet fission material itself is a tetracene molecule, which is formed of four fused benzene rings. “This is a relatively wellknown singlet fission material, which is then combined with quantum dots in the film,” outlines Dr Gray. Lead sulfide (PbS) quantum dots are being used to collect the triplet excitons, which Dr Gray says are engineered to a high level of precision. “The size of the
quantum dots determines the energy of the triplets they can accept and the energy of the photons they can emit, so they have to be of a size that they can accept the triplets that are formed through singlet fission,” he explains. “That puts a limit on how small they can be, but they can’t be too big either, or they won’t be compatible with silicon solar cells for example. So, there’s only a narrow window where quantum dots are of a suitable size.” The challenge then is to combine these inorganic quantum dots effectively with organic singlet fission molecules in a photon multiplication film, which is a complex, technically demanding task. Quantum dots are typically coated with long, carbon aliphatic chains, so that they can be suspended in solution. “They have this thick shell of oily chains, that effectively insulates them from other materials. It’s challenging to electronically couple these with the singlet fission material, in order to allow the triplets to be harvested,” says Dr Gray. These long-
EU Research
chain aliphatic molecules have been removed and replaced with a molecule that’s very similar to the singlet fission material, except that it has a group that can bind to the surface of the quantum dot. “This makes the surface of the quantum dot more compatible with the singlet fission material, while it also acts as a kind of stepping stone for the triplets to migrate into the quantum dot,” says Dr Gray. This is an important topic in Dr Gray’s research, with his focus primarily on building a deeper fundamental understanding of how the excited states move in the film and in solution. Dr Gray is using optical spectroscopy techniques to study both how the generated triplets are transferred into the quantum dot, and how efficiently they are transferred. “We have done a lot of initial studies in solution,” he says. The wider aim here is to develop a photon multiplication film as part of efforts to further improve the efficiency of solar cells, in terms of how many photons are absorbed relative to how many are emitted. “For now I have used well known singlet fission molecules like tetracene which we know can absorb a high energy photon and generate two triplets
Solar energy harvesting As a physical chemist, Dr Gray’s primary focus over the course of the project has been on fundamental research into singlet fission, yet this research also holds wider interest in solar energy harvesting. In the project, Dr Gray has collaborated closely with colleagues from both the university and a local start-up company, who are considering the potential practical applications of this research. “We always have the prospect of practical applications in mind when we discuss the next steps. This wouldn’t particularly disrupt existing manufacturing processes, as the photon multiplication film would be added on top of the solar module after it’s been produced,” he stresses. A lot of progress has been made in the project, but there is still more work to do before this research can be translated into practical applications. “We have been able to show a kind of proof-of-principle of this photon multiplication film, but the tetracene molecule and the quantum dots are slightly too low in energy to be matched with the silicon solar cells,” outlines Dr Gray.
I’m investigating the first part of singlet fission, which is about the absorption of the initial high-energy photon, which then generates two triplets with about half the energy. I’m specifically looking at the collection of these triplets in quantum dots, which ideally then emit this energy as low-energy photons. with about half the energy,” continues Dr Gray. “Then I’m specifically looking at the collection of these triplets in quantum dots, which ideally then emit this energy as low-energy photons.” The quantum dots have a slightly lower excitation energy than the triplets that are formed, which is an important consideration in terms of collecting them. When the triplets move around in the material, they get trapped in the quantum dots. “That’s why we need the quantum dots to be evenly dispersed throughout the film, so that there’s always a quantum dot close to where a triplet could be formed,” says Dr Gray. However, the quantum dots have a very large absorption spectrum, so Dr Gray says it’s also important to ensure there are not too many, so that the overall efficiency of the solar cell is not adversely affected. “Ideally the quantum dots should be completely transparent, so that the solar cell can absorb efficiently,” he explains. “We also want to achieve the right morphology, which means that the singlet fission material is not disturbed and it can efficiently undergo the singlet fission process to generate two triplets.”
www.euresearcher.com
The next step would be to move this over to another system where a singlet fission material can generate slightly higher energy triplet states. A lot of energy and attention is devoted to searching for molecules that would match effectively with silicon solar cells, with researchers both looking again at existing molecules and making quantum-chemical calculations to identify the optimal properties. “A lot of materials have been proposed,” says Dr Gray. With the project nearing the end of its funding term, Dr Gray hopes his research will prove to be an important contribution to the ongoing development of the field. “I hope to show that this new approach to incorporating singlet fission material with solar cells is viable. Traditionally, people have tried to incorporate these singlet fission materials directly with the solar cell material, and that’s turned out to be challenging,” he says. “Even though using quantum dots as an emissive material is a detour, I hope that this will be interesting to other groups and that it will lead to some useful materials in future.”
Making Dark Triplets Making Dark Triplets From Singlet Fission Bright - Improving Solar Cell Efficiencies Project Objectives
The objectives of the project is to enhance solar cell efficiencies by developing materials that can absorb one high energy photon and re-emit two photons of half the energy. The project is fundamental in nature and aims to develop and understanding of the design of these materials comprising organic molecules anchored to emissive semiconductor nanocrystals (Quantum dots).
Project Funding
The project is funded by the Swedish Research council
Project Partners
The project is funded through an International Postdoc fellowship which allows me to visit the Cavendish Laboratory, Department of Physics at Cambridge University, UK to perform the research in close collaboration with other experts on the topic. Dr. Akshay Rao is the Principle Investigator hosting me.
Contact Details
Project Coordinator, Dr Victor Gray Cavendish Laboratory University of Cambridge JJ Thomson Avenue CB3 0HE, United Kingdom E: victor.gray@kemi.uu.se W: https://rao.oe.phy.cam.ac.uk/ Rao, A., Friend, R., Harnessing singlet exciton fission to break the Shockley–Queisser limit , 2017, Nature Reviews Materials, 17063. DOI: 10.1038/natrevmats.2017.63 Gray, V., et al. Direct vs Delayed Triplet Energy Transfer from Organic Semiconductors to Quantum Dots and Implications for Luminescent Harvesting of Triplet Excitons, 2020, ACS Nano, 4224-4234. DOI:10.1021/acsnano.9b09339 Gray, V., et al. Thiol-Anchored TIPS-Tetracene Ligands with Quantitative Triplet Energy Transfer to PbS Quantum Dots and Improved Thermal Stability, 2020, J. Phys. Chem. Lett., 7239-7244. DOI: 10.1021/acs.jpclett.0c0203 Allardice, J., et al. Ligand Directed Self-Assembly of Bulk Organic-Semiconductor/Quantum-Dot Blend Films Enables Near Quantitative Harvesting of Triplet Excitons, 2020, arXiv:2009.05764; https://arxiv.org/abs/2009.05764
Dr Victor Gray
Dr Victor Gray is a post-doctoral researcher at Uppsala University, who has been working at Cambridge University since mid-2018 on an international post-doc fellowship. His main research interests lie in the photophysics of molecules and materials and their application in solar energy harvesting.
39