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Solar energy: A science with many layers

Scientists across the world, including Luxembourg, are pushing the limits of what photovoltaics can do to make using this energy source more appealing. From tandem solar cells, lost heat, new materials, studying solar cell materials on an atomic scale, bringing down cost – researchers in Luxembourg are working on it, with FNR support.

The story behind the image: A better heart for solar cells

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The heart of a solar cell: A hybrid perovskite absorber seen in nanometer resolution.

This striking image was created by Luxembourger Dr Alex Redinger with his team at the University of Luxembourg. It was made using an atomic force microscope, which allows the researchers measure the topography and electrical properties of materials down to the atomic level. In the present case, a pixel resolution of approximately 2 nanometers was achieved, which corresponds to 0.000000002 meters i.e., more than 10000 times smaller than a human hair!

“The image is a combination of two signals, the topography and an electrical property named ‘work function’. The blue regions in the image correspond to higher work functions and reflect in the present case a different material that formed at the perovskite surface, due to the processing conditions. Our work allows us to identify these different phases with nanometer precision.” – Alex Redinger

© Scanning Probe Microscopy Laboratory

Satellites, little lunar rovers, children’s toys, flashlights, calculators, buildings – the places where solar cells are used are many. Solar panels are becoming a more regular sight on rooftops in Luxembourg, in line with the national goal of 25% renewable energy by 2030. What challenges remain to make solar energy better? One is improving production and bringing the price of solar energy down, but the main challenge is increasing the amount of sunlight solar cells convert into usable energy. A scientific challenge with many sides.

Thin film solar cells consist of a variety of thin layers made of different materials that all play a part in converting sunlight into electrical power. Each layer plays a different role. The heart of the solar cell is the absorber, where the light is absorbed and converted into electric charges.

"In our work we investigate absorber layers that consist of an inorganic and an organic part, also denoted as hybrid material. The atoms in the crystal have a specific arrangement, which we call perovskite. In this image, the class of materials we investigate are therefore named hybrid organic inorganic perovskites.” -Alex Redinger

Hybrid perovskites: Promising candidates for high efficiency solar cells

Hybrid perovskites are currently getting a lot of attention from science, because they show exceptionally good properties for high efficiency solar cells. Usually, high temperature processes and clean raw materials are needed to produce high quality solar cells. This is not the case for hybrid perovskites, which exhibit very good properties, and are cheap and simple to produce.

On top of this, they can even be combined with Silicon (found e.g. in thick solar cells) and other, thin film, solar cells (the most common solar cells technology on the market). This will allow for even better solar cells in the future. The combination of two solar cells is also known as tandem solar cells.

Alex Redinger’s research group is embedded in the Physics and Materials Science department of the University of Luxembourg, the lab is one of three at the University researching elements of solar energy. Specifically, the team studies surface and interface properties of solar cell materials, using a variety of different scanning probe techniques to measure the electrical properties of the clean surfaces and interfaces at the nanoscale.

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A silicon (111) surface measured with atomic resolution.

© Scanning Probe Microscopy Laboratory

This research is supported by several FNR research grants, most notably Redinger’s FNR ATTRACT Fellowship, a 5-year 2 MEUR grant which enabled Alex Redinger to create his research group.

Tandem solar cells: The next generation solar cell

Even though the sun delivers practically unlimited energy, solar modules convert generally only about one fifth of that energy into electricity. In the quest for more efficient solar power stations, scientists are creating and studying tandem solar cells, a new type of solar cell that can harvest more sunlight thanks to multiple layers.

The main difference between standard solar cells and tandem solar cells is that tandem solar cells have more layers of semiconductors. Each layer absorbs different wavelengths (or colours) of light and converts this energy into electricity.

In a typical thin-film tandem solar cell, these semiconductors are vapour deposited as one layer on top of the other. Layering them in this way yields two solar cells that together produce a tandem effect. Much like having two cyclists on a tandem bike combine their power, layering solar cells increases their efficiency for converting sunlight to electricity.

Solar panel on house

Cells are small and the best cells reach 47% efficiency. what is technologically important are modules (the big plate we put on the roofs or in solar fields, that consist of many cells) – and they have around 20% efficiency at the moment.

More efficient solar plants equal lower cost and more appeal

The Laboratory for Photovoltaics at the University of Luxembourg, one of three labs at the institution researching how to improve different aspects of solar energy, have their sights set on bringing down the cost of solar energy by improving the efficiency of solar cells.

The team of around a dozen scientists, led by physics Professor Susanne Siebentritt investigates semiconductors and solar cells that can be used in tandem devices.

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Prof Susanne Siebentritt

© LuxTimes

“Conventional solar cells have been improved over many decades and have come close to theoretical limits. The future could be found in tandem solar cells, where we stack two different solar cells on top of each other, so that each can make better use of the sunlight and more energy is harvested.”

“The more efficient the solar cell, the smaller the area solar power stations need to cover - and the cheaper the electricity they produce. The development of thin-film tandem solar cells will lead to the installation of more solar systems and will contribute more to the fight against the climate crisis” – Prof Susanne Siebentritt

Turning lost heat into electricity

What if lost heat could be turned into electricity? It can! The Ferroic Materials for Transducers group, led by LIST’s Dr Emmanuel Defay, are working on a new FNR-funded project dedicated to the study of a new class of materials for electro-thermal energy harvesting. These materials, called ‘non-linear pyroelectrics’, are capable of converting lost heat into electricity.

The scientists will expose these materials to a variation of temperatures, while controlling their voltage. This is known as a thermodynamic cycle. The goals of the project are to get a better understanding of the main characteristics, to get the material to convert the maximum amount of heat into electricity.

Once the scientists have this knowledge, they will use it to demonstrators able to generate enough energy to run simple devices, such as an autonomous sensor. This will allow the researchers to study the energy efficiency of this effect, and how to make it even better.

Reducing environmental impact with [efficient] solar energy

Scientists in solar energy are not just working on making solar cells better, but also on how they are made: The researchers in the Laboratory for Energy Materials (LEM) focus on solar cells made with sustainable materials, and finding ways to bring down the energy needed to make them.

Delicate ingredients

Semiconductors can be found in nearly every electronic device: computers, smartphones – and solar energy – would not be possible without them. Semiconductors for solar cells are difficult to make and many techniques are expensive, as they require a lot of energy.

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Philip Dale observing parts of a solar cell with team members.

© FNR

“Finding a way to improve this process, which could ultimately bring down the cost of making a solar cell is one of the fundamental drivers of the research we do in my group.” – Chemist & physicist Prof Phillip Dale

The solar cell semiconductors Dale and his team work on are usually a solid chemical element or compound, which can conduct electricity under certain conditions.

“Semiconductors have very delicately balanced properties and a change – even on the level of removing a few atoms – can affect these properties and change everything: What we discovered with a series of semiconductors is that parts of the semiconductor were ‘evaporating’ away.” – Phillip Dale

Boosting power

The lab’s research is also focused on the preparation and characterization of semiconductor layers for use in solar cells. The goal is to understand how to convert certain ‘layers’ into high quality semiconductor materials to convert more incoming light into electrical power.

“Society should no longer use fossil fuels to power its civilization, but instead use sun and wind. Put simply, the energy that we need to generate renewably is the energy that we consume.” – Phillip Dale

Educating young people is essential for Dale, who is organising the outreach project Energy Balance, which involves school students investigating if they can live using only renewable energy resources.

“If you understand the science behind energy consumption and renewable energy generation, you can make informed, timely decisions about energy to help steer humanity to a safer future.” - Philip Dale

Training the next generation of photovoltaics researchers

All the research groups featured on this page are also working together in the framework of the Doctoral Training Unit ‘Photovoltaics: Advanced Concepts for High Efficiency’ (PACE), a joint endeavour between the University of Luxembourg and LIST, with the scientists bringing in a mix of chemistry, physics and materials research. 8 PhD researchers work on projects supervised jointly by experts to train the young scientists.

The goal all the PhD projects have in common: To create new solar cells with higher power conversion efficiencies than those currently available. The new concepts and devices will inform the development of solar power and help move the world to a renewable energy future.

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