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DTU researchers develop sustainable tandem solar cell

Friday 20 Dec 19

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Jørgen Schou
Senior Scientist, dr. scient.
DTU Fotonik
+45 46 77 47 55

Energy efficiency

Energy efficiency is a measure of the amount of energy from the sun’s rays a solar cell can transform into power.

Currently, most solar cells are made exclusively from silicon and yield an energy efficiency of about 20 per cent.

About the ALTCELL project

ALTCELL stands for ‘alternative cell’. The project has received DKK 24 million in funding from Innovation Fund Denmark and is spearheaded by DTU. It was launched on 1 January 2017 and will conclude in 2020.

The parties involved are: DTU (DTU Fotonik, DTU Nanolab, DTU Energy), Aarhus University, Nanyang Technological University in Singapore, Haldor Topsøe, Inmold, and Danish Solar Energy.

Researchers at DTU have succeeded in developing a tandem solar cell based on sustainable materials, which even beats the energy-efficiency world record for this type of tandem solar cells.

For several years, DTU researchers have worked closely with Danish catalysis company Haldor Topsøe and others to combine two types of solar cells in order to create a new so-called tandem solar cell. They have now succeeded.

“This is the first time anyone has developed a functional tandem solar cell consisting of a silicon cell covered with a thin-film cell made from the sustainable material CZTS. The advantage is that the tandem solar cell has better absorption of the energy from the sun’s rays compared to types of solar cells dominating the market today,” explains Jørgen Schou, Senior Researcher and Group Leader, DTU Fotonik, head of the research project called ALTCELL.

He emphasizes that the idea of tandem solar cells is not new—the novelty is to base them on sustainable, while at the same time competitive materials.

Sustainable thin-film cells

The new tandem solar cell was developed by combining a silicon cell with a layer of thin film consisting of a material called CZTS. Most thin-film cells today are made from the two elements cadmium, a toxic heavy metal, and tellurium, which is as rare as gold. CZTS, which is a relatively new material, consists of the substances copper, zinc, tin, and sulphur. These substances are sustainable and inexpensive because they are readily available in nature.

The majority of standard solar cells today are made from silicon, because this element is found in sand and is both inexpensive, sustainable, and able to achieve high energy efficiency, meaning that it can absorb and transform many of the sun’s rays into power. The researchers have now added a thin-film solar cell on top of this technology, enabling an increase in the absorption of solar rays and thus the power output.

Jørgen Schou explains, “The combination of CZTS and silicon is excellent, because together they absorb energy from large parts of the light spectrum.

The thin film, which is on top, absorbs the most energy-rich photons from the visible part of the light spectrum, while the rest of the light continues down to the silicon layer, which works best in the infrared spectrum.”

World record in energy efficiency

A silicon cell has an energy efficiency of about 20 per cent. By adding a CZTS thin-film cell on top, the goal is to reach a minimum of 30 per cent.

“We are trying to solve two major challenges. One is to integrate the CZTS cell with a silicon-based bottom cell. We have solved this with great success, because we have achieved an energy efficiency of no less than 3.9 per cent, which is actually a world record for this type of tandem solar cells,” explains Jørgen Schou.

“The second challenge is to improve the efficiency of the CZTS cell, and the record here was set in Australia at about 11 per cent. However, we are striving to get up to about 17-18 per cent in the long term, but we still have some way to go.”

The researchers have succeeded in constructing the tandem solar cell as one unit, but it has been a challenge to combine the two materials in the most optimal way while still preserving the properties of both. Several different methods were therefore tested, which was the process where Haldor Topsøe, among others, was involved.

“We’ve been involved in testing a scalable technique for producing CZTS thin film. The purpose was to test whether we could produce the thin film using some of the methods within our expertise. We did actually succeed, but the solar cell properties of the thin film are not yet good enough, so the project will proceed using a different technique,” says Søren Dahl, R&D Director, Advanced Materials at Haldor Topsøe.

He emphasizes that Haldor Topsøe will continue to monitor the project.

“I’m impressed at how far the ALTCELL project has come in such a short time, and our participation has been incredibly instructive for us. It will therefore be interesting to monitor the progress of the project to see, for instance, how the energy efficiency of the tandem solar cell is increased,” he elaborates.

Competitive alternative

It is essential for the new tandem solar cells to achieve even higher energy efficiency if they are to become a competitive alternative to silicon cells. And now, with the first crucial challenge solved, Jørgen Schou is optimistic.

“We have found a technique for producing the thin film, so the issue now is to upscale production in a cost-efficient way, and once we manage to optimize the techniques and increase the efficiency of the silicon cell as well as the CZTS cell, I’m convinced that this type of tandem solar cell will become an important element in the solar cell market of the future. However, it will still be some years until we have resolved the outstanding challenges.”

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