Researchers at the Helmholtz-Zentrum Berlin (HZB) have developed a new electron transport material for perovskite solar cells that improves efficiency, reduces manufacturing energy requirements and addresses a long-standing stability problem. The material, designated mCB-FMN, is based on a carborane molecule and has been patented and brought to market by Swedish company Dyenamo.
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To date, perovskite cells have relied on a layer of fullerenes, specifically C60, to extract electrons from the absorber. The drawbacks are well established, however. Significant charge carrier losses at the C60-perovskite interface, relatively high material costs and a tendency to delaminate over time, undermining cell stability. It was against this backdrop that the HZB team, working with partners at Kaunas University of Technology in Lithuania, set out to replace it.
Advantages over C60
The mCB-FMN layer can be deposited from the gas phase at lower temperatures than C60, reducing both energy consumption and thermal stress on manufacturing equipment. Measurements confirm that it extracts electrons efficiently, with lower interface losses than its predecessor. Density functional theory calculations suggest that surface defects are passivated, which may account for the improvement. Mechanical tests show stronger interfacial adhesion and better overall stability within the perovskite cell stack.
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It was found that substituting mCB-FMN for C60 in a p-i-n perovskite single-junction cell raises conversion efficiency by 1.5 percentage points in absolute terms. In perovskite-silicon tandem cells the gain reaches 2.4 percentage points, partly because the new material's lower parasitic absorption allows more light to reach the photoactive layers.
Patented and on the market
"We have developed a high-performance fullerene substitute for perovskite solar cells and used a wide range of measurements to demonstrate its advantages," says Lea Zimmermann, lead author of the study. Professor Steve Albrecht, who led the research group, noted that Dyenamo has now commercialised the material, making it broadly available. The team is also pursuing similar advances on the hole-transport side of the cell. "We are working at full speed on further new materials in this class and believe they could also transform tandem solar cells," says Albrecht.
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In a parallel study published in Joule on 9 July, the Albrecht team reported a 27.3 percent efficiency result for an all-perovskite triple-junction cell, achieved by replacing a conventional PEDOT:PSS hole transport layer with a graphene oxide/SAM double layer – an approach that also retained more than 90 percent of initial efficiency after 770 hours of continuous operation.
The carborane electron transport was published in Energy and Environmental Science, a journal of the Royal Society of Chemistry. The patent application covers mCB-FMN, its derivatives and their use in solar cells (EP 25175871.0). (TF)
