Introducing a metal fluoride layer into multilayer perovskite-silicon tandem solar cells can slow charge recombination and improve performance, MAP researchers have found.
Through King Abdullah University of Science & Technology (KAUST)
Tandem solar cells that combine perovskite and silicon-based subcells in one device are expected to capture sunlight and convert it into electricity better than their conventional single silicon analogs at a lower cost. However, when sunlight hits the perovskite subcell, the resulting electron pairs and positively charged holes tend to recombine at the interface between perovskite and the electron transport layer. Also, a mismatch between energy levels at this interface hinders electron separation in the cell. In summary, these problems reduce the maximum available operating voltage, or open circuit voltage, of the tandem cells and limit the performance of the device.
These performance problems can be partially solved by interposing a lithium fluoride layer between the perovskite and electron transport layer, which usually comprises the electron acceptor fullerene (C60). However, lithium salts liquefy easily and diffuse through surfaces, making the devices unstable. “None of the devices passed the International Electrotechnical Commission’s standard testing protocols, which prompted us to create an alternative.” says lead author Jiang Liu, a postdoc in Stefaan De Wolf’s group.
Liu, De Wolf and colleagues systematically investigated the potential of other metal fluorides, such as magnesium fluoride, as interlayer materials at the perovskite/C60 interface of tandem cells. They thermally evaporated the metal fluorides on the perovskite layer to form an ultra-thin uniform film of controlled thickness before adding C60 and top contact components. The interlayers are also highly transparent and stable, in line with the requirements for inverted solar cells.
The magnesium fluoride intermediate layer effectively promoted the extraction of electrons from the perovskite active layer while displacing C60 from the perovskite surface. This reduced charge recombination at the interface. It also improved charge transport through the subcell.
The result tandem solar cell achieved a 50 millivolt increase in its open-current voltage and a certified stabilized current conversion efficiency of 29.3 percent — one of the highest efficiencies for perovskite-silicon tandem cells, Liu says.
“Since the best efficiency is 26.7 percent for mainstream crystalline silicon-based single-junction cells, this innovative technology could deliver significant performance gains without increasing manufacturing costs,” said Liu.
To further explore the applicability of this technology, the team is developing scalable methods to produce industrial-scale perovskite-silicon tandem cells covering an area of more than 200 square centimeters. “We are also developing several strategies to obtain highly stable tandem devices that meet critical industrial stability protocols,” said Liu.
Original study: Efficient and stable perovskite-silicon tandem solar cells by contact displacement by MgFX
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