Perovskite solar cells get a fullerene-free boost

A new electron contact material developed by researchers led by Steve Albrecht at HZB could address two persistent problems in perovskite solar cells: interface losses and long-term stability. The material, called mCB-FMN, is based on a carborane molecule and is designed to replace C60 fullerene, the standard electron-transport material used today.

That matters because perovskite cells are already highly efficient. Single-junction devices can convert more than 27% of sunlight into electricity, while perovskite-silicon tandem cells have reached more than 35%. But the interface between the perovskite absorber and the C60 layer still wastes a significant share of charge carriers, and C60 can be costly and prone to delamination over time.

Working with Kaunas University of Technology (KTU) in Lithuania and other partners, the HZB team built mCB-FMN from commercially available reagents. The molecule combines a meta-carborane core with two 9-fluorenylidene malononitrile functional groups.

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Compared with C60, the new thin film can be deposited from the vapor phase at lower temperatures, reducing energy use and thermal stress during production. According to the study, it also forms a more uniform layer on the perovskite surface. Measurements including transient surface photovoltage, photoluminescence, He-I ultraviolet photoemission spectroscopy, and density functional theory calculations pointed to more effective electron transport, better energy-level alignment, and passivated surface defects.

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The team also found mechanical and fabrication benefits. Electron microscopy and in situ ellipsometry during deposition of the SnOx buffer layer suggested improved film growth, while mechanical tests showed stronger interfacial adhesion in the perovskite/ETM/SnOx stack.

Those changes translated into higher performance. Replacing C60 with mCB-FMN increased the efficiency of a single p-i-n perovskite cell by 1.5 percentage points in absolute terms. In perovskite-silicon tandem cells, the gain reached 2.4 percentage points versus the reference cell, helped in part by lower parasitic absorption.

“We have developed a very high-performance substitute material for fullerenes in perovskite solar cells, and we have demonstrated its benefits through different measurements.”

The material has already moved beyond the lab. It won the Best Scientific Content Award at the 2025 TandemPV International Workshop, a European patent application has been filed under EP 25175871.0, and Dyenamo is already selling it commercially.

The paper is “A novel carborane-based electron transport material for high-performance perovskite/silicon tandem solar cells” in Energy & Environmental Science (2026), with DOI 10.1039/d6ee01246a. Albrecht said his team is now working on more materials in the same class, with the aim of pushing tandem cells further.