Solar cells made from crystalline perovskite have come on in leaps and bounds in recent years. Now the Karlsruhe Institute of Technology (KIT) and their partners in the EU project PERCISTAND have produced a tandem cell with perovskite stacked on top of a copper indium diselenide (CIS) cell that has achieved a new record in its class of 24.9 percent efficiency.

While this is still not as high as a perovskite/silicon solar cell (around 29 percent efficiency), the advantage of the new CIS-based tandem cell is that it is highly flexible and therefore suited to mobile applications, portable devices, and even foldable devices.

"This is the highest reported efficiency for this technology and the first high efficiency ever achieved with an almost gallium-free, copper indium diselenide solar cell in tandem," says the joint project leader Doctor Marco Ruiz-Preciado from KIT's Light Technology Institute in a press release. The reduction of gallium leads to a narrow bandgap of about 1 electron volt eV, which is close to the ideal for a base cell. And because the bandgap is reduced, only a low level of bromine is needed in the perovskite topper to achieve bandgap parity between layers, thereby optimising the current flow. Moreover, with lower bromine content, the upper cell is correspondingly more stable and efficient.

"Our study demonstrates the performance potential of perovskite/CIS tandem solar cells and defines the basis for future developments that can further improve efficiency," summarises the joint project leader Professor Ulrich Paetzold. The applications for this new breed of flexible, thin-film solar system are broad from solar panels on electric vehicles to roller blinds and awnings that can collect electricity from sunlight during the day.

The research was funded by the Federal Ministry for Economic Affairs and Climate Action (BMWK).