24.9% Efficiency For Perovskite Tandem Cell

• KIT has claimed to have achieved 24.9% peak efficiency for a perovskite and CIS tandem solar cell
• The team used very little gallium to achieve a narrow band gap and low bromine perovskite to ensure efficiency and integration
• With further research improvements, efficiency of perovskite and CIS tandem cells can be increased to over 30%

A team of researchers from Germany's Karlsruhe Institute of Technology (KIT) have reported achieving 24.9% peak and 23.5% certified efficiency for a tandem solar cell comprising perovskite and copper indium diselenide (CIS) materials.

They attribute this efficiency level to the reduce amount of gallium used that led to a narrow band gap of about 1 electron volt eV, which they claim is very close to the ideal value of 0.96 eV for the bottom solar cell in a tandem.

For this research work, they used narrow-bandgap CIS solar cells for bottom tandems. To tune the band gap for efficient tandem integration, the team adjusted the bromine content of perovskite layer, using a low amount of the same to bring in efficiency and stability.

High bromine perovskites can bring in efficiency in tandem integration but can lead to voltage losses and phase instability, the team points out.

They also used an optimized antireflective coating for improved light-in coupling.

"This is the highest reported efficiency for this technology and the 1^st ever high efficiency achieved with a nearly gallium-free copper indium diselenide solar cell in a tandem," said Dr Marco A Ruiz-Preciado of Light Technology Institute (LTI). The results are part of a European Union (EU) project Percistand where the focus is on testing 4-pole tandem solar cells and prototypes for modules on glass substrates.

Tandem solar cells can be used for vehicles, portable devices and foldable or rollable devices with further improvement in efficiency. As per the team, "Our study sets the baseline for fabricating efficient perovskite/CIS TSCs, paving the way for future developments that might push the efficiencies to over 30%."

The research work titled Monolithic Two-Terminal Perovskite/CIS Tandem Solar Cells with Efficiency Approaching 25% has been published in ACS Energy Letters.

Back in September 2019, KIT along with counterparts from Center for Solar Energy and Hydrogen Research Baden-Württemberg (ZSW) launched a project called Capitano to develop perovskite tandem cells with CIGS to achieve efficiency level of over 30% (see Capitano To Explore Over 30% Efficiency For Tandem Cells).