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Research trends: Tandem solar cells – two-packs that reach new efficiency records



As part of a new series, the FNR speaks to five experts about research trends in their domain. A key research question for solar cell scientists is how to make the cells more efficient. Prof Dr Susanne Siebentritt from the University of Luxembourg explains how thin-film tandem solar cells could play a role in the solution.

Researchers worldwide have been working to improve the efficiency of solar cells for many years now. Even though the sun delivers practically unlimited energy, solar cells can generally only convert about one fifth of that energy into electricity.

In many cases, these tandem solar cells are just as efficient as single solar cells made of silicon. “But the potential of single cells is largely exhausted, while the efficiency of thin-film tandem solar cells will continue to rise. So it will be through these that we will continue to improve the electrical capacities of solar plants.

The power of two layers

A tandem solar cell consists of two layers of semiconductors that each absorb different wavelengths of light and convert 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.

These combined solar cells can namely exploit a broader spectrum of light wavelengths.

New semiconductor materials for greater efficiency

Thin film solar cells achieve efficiencies of more than 20 percent nowadays, which is an excellent energy yield in the field of photovoltaics. “This efficiency can be significantly boosted further still using novel semiconductor materials. This is currently being intensively researched,” Siebentritt assures.

Excellent materials for thin-film solar cells are the so-called chalcopyrites. At the Laboratory for Photovoltaics of the University of Luxembourg, Susanne Siebentritt and her team are developing tandem solar cells in which both cells are made of chalcopyrites. “We expect these tandem solar cells to be highly stable and efficient.”

The technology becomes more powerful – and more competitive

Given the performance gain, the new thin-film tandem solar cells will in turn increase the cost-effectiveness of solar plants.

Thin-film solar cells, by the way, get their name from being about 100 times thinner than conventional silicon solar cells. That gives thin-film solar cells the advantage of being mountable on plastic or metal foils, which means mobile devices such as smartphones could be equipped with their own power supply.

These new application prospects are greatly welcomed by architects, for example, who see huge advantages of thin-film tandem solar cells over conventional silicon solar cells. “These new kinds of power supply present an optically uniform surface and are not made up of individual cells and metal strips,” Susanne Siebentritt explains. “This makes them more appealing to look at, and so architects prefer to use them.”

Without renewable energies, society as we know it will no longer exist in 100 years

Regarding climate change, the photovoltaic expert asserts that investing in renewable energies is the only way to stop this dangerous trend.

The new thin-film tandem solar cells will give this trend a new tailwind and even bring about a new look to our society, Siebentritt is convinced. “In future, we will see ever more roofs and especially a lot more other surfaces with thin-film and other solar cells installed on them for generating electricity.”

About Susanne Siebentritt

Susanne Siebentritt is a Professor of Physics at the University of Luxembourg, where she also heads up the Laboratory for Photovoltaics, which she established in 2007.

In 2014 she received the an FNR Award in the category Outstanding Scientific Publication, together with three co-authors for the publication ‘The Consquences of Kesterite Equilibria for Efficient Solar Cells’. In 2015 she was awarded the “Grand Prix en Sciences Physique – Prix Paul Wurth” of the Luxembourgish Institut Grand Ducal.

More in this series

More on the topic of solar energy

Prof Susanne Siebentritt

Thin-film solar cells are on their way to mass production

The efficiency of tandem solar cells has continuously improved over recent years as the culmination of intensive research. In 2015, Swiss researchers were the first to succeed in producing a thin-film tandem solar cell in the laboratory that could convert sunlight into electricity more efficiently than the best single cell.

The researchers achieved this by applying a thin film of perovskite onto a thin-film solar cell made of chalcopyrite.

This combined cell converted incident sunlight into electrical current at an efficiency of 20.5 percent, while the chalcopyrite solar cell on its own was reaching an efficiency of 18 percent.

Because the scientists managed to produce this thin-film tandem solar cell at relatively low temperatures and using cheaper materials, their development marks an important step on the path towards mass production of more efficient solar cells.