How perovskite solar cells age under temperature stress

Researchers from the Technical University of Munich (TUM), together with partners from Karlsruhe Institute of Technology (KIT), Deutsches Elektronen-Synchrotron (DESY), and KTH Royal Institute of Technology in Sweden, have examined why perovskites in tandem solar cells lose performance under temperature cycling. Two recently published papers show, on the one hand, how rapid temperature cycles affect the crystal structure, and on the other, which organic molecules can stabilize perovskite structures.

The first study focuses on perovskites as typically used in perovskite-silicon tandem solar cells. The researchers used cells with an efficiency of 24.31%. The temperature was varied at a rate of 10 C per minute between 5 C and 85 C.

The authors note that these rapid cycles do not correspond to standard certification conditions. IEC testing uses significantly longer cycles of 100 degrees Celsius per hour. The method used in the experiment was intended to accelerate aging processes for material screening.

Wide-angle X-ray scattering and photoluminescence measurements were used for the analysis. These allowed the researchers to observe in real time how the crystal lattice expands and contracts under temperature changes and how photovoltaic parameters change in parallel.

The most important observation of the first paper is a two-stage degradation. First, a pronounced “burn-in phase” occurs. In this initial phase, the cells lost around 60% of their relative performance under rapid solar-thermal cycling. This was followed by a slower degradation phase, during which the parameters partially tracked the temperature profile.

The authors identify the cause as a combination of temperature-induced mechanical stress, phase transformations, and increasing non-radiative recombination. In simple terms, the perovskite layer expands differently under temperature changes than the adjacent layers and the substrate, creating stress in the material. These stresses alter the structure and degrade electrical properties.

Lead author Kun Sun from the TUM Chair of Functional Materials said the performance loss is driven by competing forces within the material at the microscopic level, where internal stresses build up and alter the structure, ultimately degrading performance.

Notably, degradation under these cycling conditions was largely independent of the passivation strategy tested, according to the study. Uncoated cells, cells with EDAI2 passivation, and cells with a dual passivation of 3-F-PEAI and EDAI2 were examined. While passivation initially improved cell efficiency, it did not prevent thermal degradation. The authors therefore conclude that common passivation approaches do not automatically lead to better thermal operational stability.

Another result relevant for tandem applications: perovskite-silicon tandem cells showed improved temperature robustness at lower temperatures; after more than 200 minutes under thermal cycling, 94% of the original efficiency remained. This suggests that integration into a tandem configuration can change behavior.

The second study explored potential solutions. In the experiment, the perovskite layer was supplemented with organic molecules intended to better buffer thermally induced expansion. Two spacer cations were compared: butylammonium (BA) and 1,4-phenylenedimethylammonium (PDMA).

The conditions were similar: 5 C to 85 C with a temperature change rate of 10 C per minute and a cycle duration of 15 minutes. BA-based layers showed clear phase separation and structural degradation after just three cycles. In contrast, PDMA-based layers were significantly more stable and remained structurally largely intact.

Prof. Peter Müller-Buschbaum of the TUM School of Natural Sciences argued that the future of photovoltaics is increasingly centered on tandem architectures. He said that by understanding the underlying microscopic mechanisms, researchers are helping enable a new generation of solar modules that combine high efficiency with durability sufficient for decades of outdoor operation.

The research results have appeared in two scientific journals. The first paper, “Insights into the operational stability of wide-bandgap perovskite and tandem solar cells under rapid thermal cycling,” was recently published in Nature Communications. The second study, “Halide Segregation in Wide-Bandgap Quasi-2D Perovskites under Rapid Thermal Cycling,” was published in ACS Energy Letters.