A team of scientists from KAUST Solar Centre has found that inserting a metal fluoride layer on multi-layered perovskite-silicon tandem solar cells can stall charge recombination and enhance performance.

When sunlight strikes the perovskite sub-cell, the resulting pairs of electrons and positively charged holes recombine at the interface between perovskite and the electron-transport layer. Also, a mismatch between energy levels at this interface hinders electron separation within the cell. These lower the maximum operating or open-circuit voltage of the tandem cells and limit device performance.

The research has found that these issues can partially be solved by introducing a lithium fluoride layer between the perovskite and electron-transport layer, which usually comprises the electron-acceptor fullerene (C60).

However, a problem with this is lithium salts readily liquify and diffuse through surfaces, thereby making the devices unstable.

“None of the devices have passed the standard test protocols of the International Electrotechnical Commission, prompting us to create an alternative,” lead author Jiang Liu, a postdoctoral fellow in the KAUST Solar Center, said.

The research team is developing high-efficiency perovskite/silicon tandem solar cells using interface and composition engineering.

Metal Fluoride Interlayer

The team has systematically investigated the potential of other metal fluorides as interlayer materials.

An interlayer of magnesium fluoride was found to effectively promote electron extraction from the perovskite active layer while displacing C60 from the perovskite surface. It reduced charge recombination at the interface while enhancing charge transport across the sub-cell.

“By inserting an ultrathin fluoride-based layer at the sunward-facing perovskite/C60 interface, at the electron-selective top contact, we demonstrate that the charge transport and recombination interfaces can be successfully tuned while maintaining a high transparency for incoming light,” De Wolf of KAUST said.

“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% — one of the highest efficiencies for perovskite–silicon tandem cells,” Liu added.

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

To test the applicability of this technology, the team is developing scalable methods to produce industrial-scale perovskite–silicon tandem cells with areas exceeding 200 square centimeters.

Earlier this year, scientists from the U.S. National Renewable Energy Laboratory developed a new tin-lead perovskite tandem cell merging different layers of two chemical compounds achieving a power conversion efficiency (PCE) of 25.5%.

Researchers from the ARC Centre of Excellence in Exciton Science recently demonstrated power conversion efficiencies of 15.5% and 4.1% in solar glass, with varying prototype semiconductor solar cells made of perovskite materials, with a visible transmittance percentage of 20.7% and 52.4%, respectively.