Soft materials for solar energy conversion
SOFT-PHOTOCONVERSION Solar Energy Conversion without Solid State Architectures: Pushing the Boundaries of Photoconversion Efficiencies at Self-healing Photosensitiser Functionalised Soft Interfaces
Solar energy has an important role to play in meeting growing global demand for energy, yet conventional methods of making solar cells have some significant limitations. We spoke to Dr Micheál Scanlon about his work in investigating a new approach which could lead to the emergence of a new type of solar conversion device. The conventional method of making solar cells is by using inorganic materials to make solid state architectures, through which light is harvested and converted into chemical energy. However, inorganic materials are expensive and production is energy-intensive, while this approach has other shortcomings which limit the effectiveness of solar cells. “When a light is shone on a material an exciton particle is created. It then has to be separated into a positive charge and a negative charge, and this leads to different problems. For example, if there is an impurity in the material, the charge may get stuck,” explains Dr Micheál Scanlon, a lecturer in chemistry at the University of Limerick. As the Principal Investigator of the Soft Photoconversion project, Dr Scanlon is now exploring a new approach to solar energy conversion, based on the use of a liquid-liquid interface. “We’re trying to convert solar energy without using solid materials,” he says.
Water-oil interface This research is built on Dr Scanlon’s expertise in controlling an electric field at a water-oil interface. Photosynthesis is a good example of nature efficiently converting light energy into stored chemical energy, so Dr Scanlon draws inspiration in his research from the natural world. “In a biological cell you have a fatty, oil-like membrane in the middle, and water on either side. We’re trying to mimic a membrane-cell structure in the water-oil interface,” says Dr Scanlon. The oil used in the interface has to be extremely hydrophobic, meaning that it is incapable
of mixing with water. “The oil that we have picked is very immiscible with water, and the oil molecules only mix with the water in a 1 nanometre region, meaning the interface is just a nanometre thin,” outlines Dr Scanlon. The photoproducts are separated at the interface based on their affinity to water, with one side of the interface
the system is that everything happens at the liquid-liquid interface. “Any molecule more than 2 nanometres away from the interface is lost, as electrons can travel only a tiny distance between molecules. So the dye on the interface, the molecule in the water, and the molecule in the oil, all have to be within a few nanometers (at most) of
We shine a light on the dye at the liquid-liquid interface. The dye uses that energy to effectively move an electron from the low-energy molecule in the oil to the high energy molecule in the water. very hydrophilic, while the other is very hydrophobic. Researchers take advantage of this to convert light energy into chemical energy, using a dye. “We shine light on the interface, and an electron is transferred from the molecule in the oil to the molecule in the water,” explains Dr Scanlon. One major issue to consider here is back electron transfer, which Dr Scanlon says can limit conversion efficiency. “You need inputted energy to move the electron from the low-energy molecule to the high-energy molecule,” he outlines. “We shine a light on the dye at the liquid-liquid interface. The dye uses that energy to effectively move an electron from the low-energy molecule in the oil to the high energy molecule in the water.” A lot of attention in the project is now focused on modifying the interface to improve the efficiency of the energy conversion process. The key point about
each other when the light hits,” explains Dr Scanlon. The concentration of dye at the interface is an important factor in solar conversion efficiency, so a lot of attention in the project has focused on optimising these strategies of dye-sensitising the liquid-liquid interface. “The photoconversion efficiency increases linearly with the amount of dye on the liquid-liquid interface, so the more dye we can concentrate there, the better the performance of our system” says Dr Scanlon. This is a very novel approach to solar conversion, so Dr Scanlon and his colleagues have created a number of their own customised techniques during the project as they aim to improve photoconversion efficiency. A range of techniques have been used to characterise a liquid-liquid interface, including electrochemical, spectroscopic and surface tension measurements methods. “We’ve also just started doing confocal
Photo by Andreas Gücklhorn
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EU Research
Project Objectives
The objectives of Dr Scanlon’s research group are to: • develop methodologies to functionalize soft interfaces with photoactive materials, e.g., dyes or semiconductors, • develop in situ methodologies to characterise materials at soft interfaces, • use these photoactive soft interfaces in all-liquid-based solar cells, • optimize the efficiency of these novel solar cells.
Project Funding
The concept of “SOFT-PHOTOCONVERSION”: The oil phase contains a molecule capable of being oxidised and reduced, but of low energy. The water phase also contains such a molecule, but of much higher energy. To drive electron transfer “up-hill” from the molecule in the oil to the molecule in the water, we trap solar energy using a layer of concentrated dye at the interface. This input of energy from light is converted to chemical energy in the form of a reduced molecule in the water and an oxidised molecule in the oil.
raman spectroscopy at the liquid-liquid interface. We’re using these techniques to look at our dye at the liquid-liquid interface,” outlines Dr Scanlon. One of the principal goals of the project has been achieved, namely maximising the concentration of dye at the interface, now Dr Scanlon is looking towards the next steps. “We plan to move forward and optimise the kinetics of the photo-electrochemistry at the liquid-liquid interface,” he says. There are a lot of kinetic elements to consider here, and it’s important to gain deeper insights into each of the steps involved. With the project approaching the halfway point of its funding term, Dr Scanlon plans to make further calculations over the coming year. “How fast is electron transfer? How fast is re-combination? What’s the ratio of the electron transfer to recombination?” he outlines. These are important issues in terms of conversion efficiency. “We aim to maximise the rate of electron transfer. We also want to maximise the rate of photoproduct separation, which is essentially the opposite of recombination,” continues Dr Scanlon. “If the charge separates then you’ve effectively converted light energy into chemical energy. But if they recombine then this is undone.”
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Looking to the future The long-term goal would be to apply this approach more widely in solar energy conversion, an objective very much in line with the EU’s Renewable Energy Directive and its 2050 Energy Strategy, which set ambitious goals around future provision of energy from renewable sources. While this technology is not yet ready for wider application, Dr Scanlon is considering how it could be used in traditional solar cells. “One approach is through selfassembly at the liquid-liquid interface, where the molecules use the water-oil interface as a template. When they adsorb at the interface, each molecule will adsorb in a specific orientation,” he says. “The point with the system is that you have a set of molecules in the oil, and a set of molecules in the water. When you pour them out everything self-assembles and you don’t have to do anything further. We’re really working on that intensively.” This research is largely exploratory at this stage, rather than being directly concerned with practical applications. Research into renewable energy is widely recognised as a major priority however, and the project’s work provides strong foundations for further development. “A future project would take this proof-of-concept, develop strategies to make the dye more concentrated, and then really start engineering them with respect to useable solar cell devices,” says Dr Scanlon.
Dr Scanlon’s research is funded by a European Research Council Starting Grant (agreement no.716792) and Science Foundation Ireland Starting Investigator Research Grant (grant number 13/SIRG/2137).
Contact Details
Dr Micheál Scanlon Department of Chemical Sciences AD3-017 Analog Devices Building Bernal Institute University of Limerick (UL) Limerick, Ireland T: +353-61-237760 E: micheal.scanlon@ul.ie W: https://www.ul.ie/research/blog/ulresearcher-awarded-€15m-pioneering-solarenergy-research Dr Micheál Scanlon
Dr Micheál D. Scanlon is a Principal Investigator in the Bernal Institute, and lecturer in the Department of Chemical Sciences, at the University of Limerick. His research involves nanomaterial self-assembly and electrochemistry at immiscible liquid-liquid or “soft” interfaces for solar energy conversion, electrocatalysis and sensor development.
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