Record-breaking solar cells feature self-assembling molecules

Tandem perovskite-CIGS solar cells, produced as a result of the collaboration between Lithuanian and German researchers, have reached an efficiency of 23.26% — said to be a world record value in this type of cell. One reason for the success lies in the cell’s intermediate layer of organic molecules: they self-assemble to cover even rough semiconductor surfaces.

Tandem solar cells of various configurations today are on the agenda of many research teams around the world, as the technology seeks to overcome the limits of single-cell efficiency. Moreover, in the production of a thin-film tandem solar cell, flexible substrates can be used, which allows interesting solutions for contemporary technology and design applications.

Physicists at Helmholtz-Zentrum Berlin (HZB), Germany, designed tandem solar cells that combine perovskite and copper-indium-gallium-selenide (CIGS). The contact layers between these two semiconductors consist of carbazole-based organic molecules, functionalised with phosphonic acid groups, which arrange themselves into what are known as self-assembled monolayers (SAMs).

The materials used for SAMs were developed by researchers at Kaunas University of Technology (KTU), Lithuania. KTU chemists have synthesised molecules assembling themselves into monolayers, which can evenly cover any surface — including rough surfaces of the CIGS solar cells.

“Usually the problem with fabrication of the perovskite-CIGS tandems is the rough surface of the bottom cell, which is difficult to cover evenly by conventional solution-based processes,” said KTU PhD student Artiom Magomedov. “Our method allows forming the electrode layer as thin as 1–2 nm, covering all the surface. It is a cost-efficient and effective process — the molecules are deposited on the surface from diluted solutions and the molecule-thick selective layer is formed.”

The used materials are strikingly simple and robust, which also allows them to be scaled up to industrial levels.

The KTU chemists have been studying the use of the self-assembling molecules to form the electrode layer for a couple of years. The material, synthesised at KTU, was applied in the production of a functioning solar cell with just a monolayer-thick selective contact by HZB physicists. They could produce a monolithic tandem solar cell from perovskite and CIGS that achieved an officially certified efficiency of 23.26%, which is currently a world record. The tandem cell also has an active area of 1 cm², whereas perovskite CIGS tandem cells have so far been significantly smaller.

“Thin-film tandem solar cells are the future of solar energy, as they are cheaper and potentially much more efficient,” said Professor Vytautas Getautis, the head of the KTU research group behind the invention.

The world-record tandem solar cell was presented at EU PVSEC, the European Photovoltaic Solar Energy Conference and Exhibition, on 11 September. Two patents have now been filed for the layers of the cells.

Image caption: The record-breaking solar cell has an intermediate layer of organic molecules: they self-assemble to cover even rough semiconductor surfaces.

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