Cheaper, more efficient perovskite solar cells

European scientists have synthesised a molecule that could assist with selective layer formation in perovskite solar cells. Described in the journal Advanced Energy Materials, the molecule assembles itself into a monolayer which can cover a variety of surfaces and can function as a hole-transporting material in a perovskite solar cell.

Perovskite-based solar cells are at the forefront of emerging photovoltaics, already competing in efficiency against well-established solar technologies used in solar panels around the world. An important step towards mass production of this new generation of solar cells is the development of efficient selective contact layers that would be compatible with the deposition of perovskite layers on various substrates.

Spin coating and vapour deposition are the two main methods which are currently being used for the formation of the layers in perovskite solar cells. Spin coating involves dripping liquid solution on spinning surfaces, but during the process large quantities of the material are lost. Vapour deposition, meanwhile, needs high temperatures and complex vacuum technologies, and not all molecules are suitable for evaporating.

Now, chemists from Lithuania’s Kaunas University of Technology (KTU) have synthesised a molecule that assembles itself into a monolayer and can evenly cover any oxide surface — including the textured surfaces of the silicon solar cells used in tandem architectures.

“It’s not polymer, but smaller molecules, and the monolayer formed from them is very thin,” said Ernestas Kasparavičius, a PhD student at KTU. “This, and the fact that the monolayer is being formed through dipping the surface into the solution, makes this method much cheaper than the existing alternatives. Also, the synthesis of our compound is a much shorter process than that of the polymer usually used in production of perovskite solar cells.”

The synthesised material was subsequently tested by physicists at Helmholtz-Zentrum Berlin (HZB), headed by Dr Steve Albrecht in collaboration with KTU doctoral student Artiom Magomedov. Together, they successfully used the material as a hole-transporting layer in perovskite solar cells.

“In our laboratory in Kaunas, we studied use of the self-organising molecules to form the electrode layer as thin as 1–2 nm, evenly covering all the surface,” Magomedov said. “During my internship in Berlin I was able to apply our material and to produce a first functioning solar element with just a monolayer-thick selective contact.”

The self-assembling monolayer technique not only results in low material consumption, but also high efficiency — the element’s power conversion efficiency was close to 18%, which is exceptionally high for a new technology. Also, when the monolayer is used as a hole-transporting layer in perovskite cells, no additives are needed to improve the performance of the cells. This could significantly improve the lifespan of the elements.

Since this approach to perovskite solar cells has never been considered before and can potentially play a role in industrial processes, the researchers have filed a patent application on the molecule and its use. The KTU scientists are also synthesising new materials for monolayer formation, with the first tests of the optimised materials at HZB resulting in solar cells with greater than 21% efficiency.

Image caption: The molecule, synthesised by the KTU chemists, assembles itself into a monolayer which can cover a variety of surfaces and can function as a hole-transporting material in a perovskite solar cell.

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