Considerable performance gains in perovskite–silicon tandem solar cells (such as that pictured here) can be achieved by adding a magnesium fluoride interlayer. Credit: © 2022 KAUST; Erkan Aydin

An extra metal fluoride layer enables charge separation and boosts performance in perovskite–silicon tandem solar cells.

Inserting a metal fluoride layer in multilayered perovskite–silicon tandem solar cells can stall charge recombination and enhance performance, King Abdullah University of Science & Technology (KAUST) researchers have found.

Tandem solar cells that combine perovskite and silicon-based subcells in one device are expected to better capture and convert sunlight into electricity than their conventional single-junction silicon competitors. And they are anticipated to do so at a lower cost. However, when sunlight strikes the perovskite subcell, the resulting pairs of electrons and positively charged holes tend to recombine at the interface between perovskite and the electron-transport layer. Additionally, a mismatch between energy levels at this interface hinders electron separation within the cell. These issues together reduce the tandem cells’ open-circuit voltage, or maximum operating voltage, which restricts device performance.

By adding a layer of lithium fluoride between the perovskite and the electron-transport layer, which typically comprises the electron-acceptor fullerene (C₆₀), these performance concerns may be partly resolved. However, the devices become unstable because lithium salts easily liquefy and diffuse through surfaces. Lead author Jiang Liu, a postdoc in Stefaan De Wolf’s group, states, “None of the devices have passed the standard test protocols of the International Electrotechnical Commission, prompting us to create an alternative.”

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 ultrathin uniform film with controlled thickness before adding C₆₀ and top contact components. The interlayers are also highly transparent and stable, in line with the inverted p-i-n solar cell requirements.

The magnesium fluoride interlayer effectively promoted electron extraction from the perovskite active layer while displacing C₆₀ from the perovskite surface. This reduced charge recombination at the interface. It also enhanced charge transport across the subcell.

The resulting tandem solar cell achieved a 50 millivolt increase in its open-current voltage and a certified stabilized power conversion efficiency of 29.3 percent — one of the highest efficiencies for perovskite–silicon tandem cells, Liu says.

“Considering that the best efficiency is 26.7 percent for mainstream crystalline silicon-based single-junction cells, this innovative technology could bring considerable performance gains without adding to the cost of fabrication,” Liu says.

To further explore the applicability of this technology, the research team is developing scalable methods to produce industrial-scale perovskite–silicon tandem cells with areas exceeding 200 square centimeters (31 square inches). “We are also developing several strategies to obtain highly stable tandem devices that will pass the critical industrial stability protocols,” Liu says.

Reference: “Efficient and stable perovskite-silicon tandem solar cells through contact displacement by MgF_x” by Jiang Liu, Michele De Bastiani, Erkan Aydin, George T. Harrison, Yajun Gao, Rakesh R. Pradhan, Mathan K. Eswaran, Mukunda Mandal, Wenbo Yan, Akmaral Seitkhan, Maxime Babics, Anand S. Subbiah, Esma Ugur, Fuzong Xu, Lujia Xu, Mingcong Wang, Atteq ur Rehman, Arsalan Razzaq, Jingxuan Kang, Randi Azmi, Ahmed Ali Said, Furkan H. Isikgor, Thomas G. Allen, Denis Andrienko, Udo Schwingenschlögl, Frédéric Laquai, Stefaan De Wolf, 23 June 2022, Science.DOI: 10.1126/science.abn8910