Perovskite solar cells: Predictions of long-term stability
The HZB-team runs an outdoor laboratory in Berlin where a great variety of solar cells are exposed for months and years to real-world conditions. © Industriefotografie Steinbach/ HZB
Three key degradation mechanisms were observed in PSCs that had undergone natural ageing in HZB’s outdoor laboratory in Berlin. The most significant of these is phase segregation (left), whereby the compositional changes in perovskite material lead to the formation of circular domains a few micrometres in diameter. The other two mechanisms are copper corrosion (middle) and edge patterns (right), which are primarily related to cell design. © HZB
Reliable statements about the long-term stability of perovskite solar cells are still difficult to make. However, a new study by Dr Carolin Ulbrich’s team, published in the renowned journal Joule, highlights which methods are useful for this purpose and identifies areas where further research is needed.
Perovskite solar cells (PSCs) could conquer the mass market within a few years, perhaps even being produced in Europe. Their large-scale production is highly cost-effective, and unlike silicon solar cells, their production is less energy intensive. However, perovskite solar cells ideally need to achieve decades-long warranties, which remains a challenge. To assess their long-term stability, various test methods are used to accelerate ageing. But how accurately do these methods reflect the actual degradation processes? A new study in Joule by a team led by Dr Carolin Ulbrich (HZB) and Andreas Bartelt (HTW Berlin) now answers this question.
Natural degradation mechanisms
In the study, naturally aged perovskite solar cells were compared with 'artificially aged' perovskite solar cells. First, the team identified three key degradation mechanisms in PSCs that had undergone 20 months of natural ageing in HZB’s outdoor laboratory in Berlin under real-world conditions. The most significant of these is phase segregation, whereby the compositional changes in perovskite material lead to the formation of circular domains a few micrometres in diameter. The other two mechanisms are copper corrosion and edge patterns, which are primarily related to cell design. In the laboratory, they subjected newly produced PSCs to various accelerated ageing processes and investigated how well the three observed degradation phenomena could be replicated.
How to accelerate the aging process?
A very common method to accelerate the aging processes is to keep the samples at elevated temperatures (65-85 °C). In a previous study, the group demonstrated that this method triggered an additional degradation mechanism that is not observed at lower temperatures or in outdoor-aged samples. As a result, the group turned to alternative methods for accelerating degradation.
Aging under 2.3 suns
Degradation phenomena can also be intensified by increasing the light intensity or by varying the electrical bias. ‘Increasing the light intensity from one sun to 2.3 suns accelerates all three degradation mechanisms while preserving the spatial trends observed outdoors, thus enabling ageing in fast-forward,’ says Ulas Erdil, first author of the study. However, whilst different bias voltages (aging under open-circuit condition) also promote phase segregation, they simultaneously affect the spatial extent of copper corrosion and the formation of edge patterns, meaning that the degradation is no longer representative of the real-world degradation.
A useful tool
As this study further shows, reliable lifetime predictions through accelerated ageing tests currently remain challenging. ‘However, they are useful tools for the rapid screening of new materials or cell designs and can therefore help to advance the development of perovskite technology,’ says Erdil.
‘We do not yet have the perfect solution for reliably predicting long-term stability,’ emphasises Carolin Ulbrich. ‘But we are one step closer; we now know that more intense light is one key parameter for accelerating the ageing process.’
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