Fraunhofer ISE reveals errors in common UV tests for ‘TOPCon’ solar modules...

14 Apr 2025

...while EPFL boosts efficiency of perovskite solar cells with rubidium stabilization.

Reports of UV-induced degradation in TOPCon-based photovoltaic modules are currently preoccupying the PV industry. The Fraunhofer Institute for Solar Energy Systems (ISE) has been therefore investigating the validity of common testing methods.

Recent results from its comparative indoor and outdoor tests show that today’s “standard UV tests” significantly over-estimate the degradation of TOPCon silicon PV modules compared with the actual performance reduction. To achieve more meaningful test results in the laboratory, the PV modules must be stabilized after UV irradiation and before performance measurement, ISE stated.

The institute presented its comprehensive analyses of the recovery dynamics of TOPCon modules at the 15th Annual SiliconPV Conference, last week in Oxford, UK.

Daniel Philipp, Head of Module Characterization and Reliability at ISE, commented, “For results that are more closely related to degradation in the field, the PV modules must be stabilized after testing. This is the only way to distinguish UV-sensitive from less sensitive module types and evaluate them comparably.

“Unfortunately, many module types of the current generation of commercial TOPCon PV modules react sensitively to UV irradiation. Field returns and comparisons between laboratory-aged and field-aged modules also confirm this. However, the degradation rate does not appear to be as drastic as was previously assumed. We recommend that users test PV modules according to the latest findings. Researchers need to further analyze the phenomenon to more accurately predict the long-term effects of UV-induced degradation on module yield,” Philipp said.

ISE investigations indicate that UV irradiation during testing destabilizes the modules to such an extent that they significantly lose efficiency during dark storage after UV exposure. Subsequent exposure to sunlight, however, leads to a significant recovery effect. Field tests at Fraunhofer ISE's Outdoor Performance Lab and analyses of “field returns” in the institute’s CalLab PV Modules from systems with TOPCon modules indicate that this stabilization process yields degradation measurements that are significantly closer to the values actually measured in practice.

Some PV modules showed hardly any degradation after UV testing at 60 kilowatt hours per square meter, which roughly corresponds to one year’s UV exposure in Germany, and subsequent stabilization under sunlight. Other modules still showed significant performance declines of up to 5 percent even after stabilization. Overall, however, the degradation appears to be significantly less drastic than the standard UV tests suggest.

EPFL boosts efficiency

Researchers at EPFL, Lausanne, Switzerland, have found a way to significantly reduce energy loss and boost efficiency of perovskite solar cells by incorporating rubidium using lattice strain – a slight deformation in the atomic structure that helps keep rubidium in place.

Perovskite solar cells, particularly those used in tandem configurations, rely on wide-bandgap (WBG) materials—semiconductors that absorb higher-energy ("bluer") light while letting lower-energy (redder) light pass through—to maximize efficiency. However, wide-bandgap perovskite formulations often suffer from phase segregation, where different components separate over time, which causes a decline in performance.

One solution is to add rubidium (Rb) to stabilize WBG materials, but there is a catch, notes EPFL: Rb tends to form unwanted secondary phases, which reduces its effectiveness in stabilizing the perovskite structure.

Scientists led by Lukas Pfeifer and Likai Zheng in the Michael Grätzel group at EPFL have found a way to force the Rb to stay where it is needed. By utilizing the “lattice strain” of the perovskite film, they managed to incorporate Rb ions into the structure, which prevented the unwanted phase segregation. This novel approach not only stabilizes the WBG material but also improves its energy efficiency by minimizing non-radiative recombination, which is associated with energy loss.

Verifying and fine-tuning the approach

To confirm and understand this effect, the team used X-ray diffraction to analyze structural changes, solid-state nuclear magnetic resonance to track the atomic placement of Rb, and computational modeling to simulate how the atoms interact under different conditions. These techniques provided a detailed picture of how strain stabilized Rb incorporation.

Besides lattice strain, they also found that introducing chloride ions is key to stabilize the lattice by compensating for the size differences between the incorporated elements. This ensured a more uniform distribution of ions, reducing defects and improving overall material stability.

The result is a more uniform material with fewer defects and a more stable electronic structure. The new perovskite composition, enhanced with strain-stabilized Rb, achieved an open-circuit voltage of 1.30 V—an impressive 93.5% of its theoretical limit. This represents one of the lowest energy losses ever recorded in WBG perovskites. Moreover, the modified material showed improved photoluminescence quantum yield (PLQY), indicating that sunlight was being more efficiently converted into electricity.