An additional layer of metal fluoride permits charge separation and improves the performance of perovskite–silicon tandem solar cells.

Addition of a metal fluoride layer to multilayered perovskite–silicon tandem solar cells can prevent charge recombination and improve performance, according to researchers at King Abdullah University of Science and Technology. Perovskite-based and silicon-based subcells combined in a single device are expected to capture and convert sunlight more efficiently than single-junction silicon competitors in tandem solar cells. Additionally, it is anticipated that they will accomplish this at a more affordable cost. When light from the sun penetrates the perovskite subcell, electrons and highly reactive holes have a tendency to recombine at the interface between the perovskite and the electron-transport layer. In addition, a mismatch in energy levels at this interface impedes the separation of electrons within the cell. Together, these difficulties diminish the open-circuit voltage or maximum operating voltage of tandem cells, hence limiting device performance.

If you place a layer of lithium fluoride between the electron-transport layer and the perovskite, which typically consists of the electron-acceptor fullerene (C60), you might be able to ameliorate some of these performance issues. However, the devices become unstable due to the liquification and diffusion of lithium salts on surfaces. Lead author Jiang Liu, a postdoc in Stefaan De Wolf’s lab, explains, “None of the devices have passed the International Electrotechnical Commission’s conventional test standards, pushing us to develop an alternative.”

Liu, De Wolf, and colleagues studied systematically the viability of additional metal fluorides, including magnesium fluoride, as interlayer materials at the perovskite/C60 interface of tandem cells. Before adding C60 and the top contact components, they used thermal evaporation to remove the metal fluorides from the perovskite layer. This allowed them to build an ultrathin sheet with a uniform thickness that could be controlled. In inverted p-i-n solar cells, the interlayers are required to be both transparent and stable in order to conform to the criteria.

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While C60 was removed from the perovskite surface, the magnesium fluoride interlayer facilitated electron extraction from the active layer. Reduced recombination at the interface as a result of this. Charge transport was also improved by this method.

The resulting perovskite–silicon tandem solar cell increased its open-current voltage by 50 millivolts and had a certified stabilized power conversion efficiency of 29.3 percent, according to Liu.

While conventional single-junction crystalline silicon-based cells have an efficiency of 26.7%, Liu believes that this new technology could provide significant performance enhancements without increasing fabrication costs.

Scalable methods for industrial-scale production of perovskite–silicon tandem cells with surfaces more than 200 square centimeters are being developed by the research team in order to further explore the potential of this new technology (31 square inches). “We are also developing a number of ways to generate very stable tandem devices that will pass the crucial industrial stability requirements,” Liu adds.

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