Quantum innovation advances low-cost alternative solar technology

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A team of researchers from the University of Toronto’s Faculty of Applied Science and Engineering have harnessed quantum mechanics to optimize the active layer of a device known as the inverted perovskite solar cell – a technology that could one day result in consumer solar cells only a fraction of those currently on the market.

Today, virtually all commercial solar cells are made from high-purity silicon, which requires considerable energy to produce. But researchers around the world are experimenting with alternative solar technologies that could be manufactured and installed with less energy and at a lower cost.

One such alternative, which is being studied in the Sargent Group laboratory, is known as perovskite. The power of perovskite materials comes from their unique crystalline structure, which allows them to absorb light in a very thin layer and convert it efficiently into electricity.

“Perovskite crystals are made from a liquid ink and applied to surfaces using technology that is already well established in the industry, such as roll-to-roll printing,” explains Hao Chenpostdoctoral researcher in Sargent’s lab and one of four co-lead authors of a new paper published in Nature Photonics.

“For this reason, perovskite solar cells have the potential to be mass-produced at a much lower energy cost than silicon. The challenge is that currently, perovskite solar cells lag behind traditional silicon cells in terms of stability. In this study, we sought to fill this gap.

Chen, with his co-lead authors – PhD candidate Sam Teale and post-docs Ben Chen and Yi Hou – use a strategy based on an inverted solar cell structure.

In most prototype perovskite solar cells, electrons exit through a negative electrode in the bottom layer of the cell, with the “holes” they leave behind exiting through a positive electrode at the top.

Reversing this arrangement allows the use of alternative fabrication techniques and previous research has shown that these can improve the stability of the perovskite layer. But change comes at a performance cost.

“It’s difficult to get good contact between the perovskite layer and the top electrode,” says Chen. “To solve this problem, researchers usually insert a passivation layer made up of organic molecules. This works very well in the traditional orientation, because the “holes” can pass through this layer of passivation. But the electrons are blocked by this layer, so when you flip the cell, it becomes a big problem.

The team overcame this limitation by taking advantage of quantum mechanics – the physical principle that states that the behavior of materials at very small length scales is different from what is observed at larger scales.

“In our solar cell prototypes, the perovskites are confined to an extremely thin layer – only one to three crystals in height,” Teale explains. “This two-dimensional shape allows us to access properties associated with quantum mechanics. We can control, for example, what wavelengths of light perovskites absorb or how electrons move through the layer.

The team first used a chemical technique established by other groups to produce a two-dimensional perovskite surface on top of their solar cell. This allowed the perovskite layer to perform the passivation on its own, completely eliminating the need for the organic layer.

To overcome the electron blocking effect, the team increased the thickness of the perovskite layer from one crystal in height to three. Computer simulations had shown that this change would change the energy landscape enough to allow electrons to escape into an external circuit, a prediction confirmed in the laboratory.

The power conversion efficiency of the team’s cells was measured at 23.9%, a level that did not fade after 1,000 hours of operation at room temperature. Even subjected to an industry-standard accelerated aging process at temperatures up to 65°C, performance declined by only 8% after over 500 hours of use.

Future work will focus on increasing cell stability, including at even higher temperatures. The team would also like to build cells with a larger surface area, as current cells are only about five square millimeters.

Still, the current results bode well for the future of this alternative solar technology.

“In our paper, we compare our prototypes to traditional and inverted perovskite solar cells that have recently been published in the scientific literature,” Teale explains.

“The combination of high stability and high yield that we have achieved really stands out. We must also keep in mind that perovskite technology is only a few decades old, while silicon has been worked on for 70 years. There are still many improvements to come. »

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