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Modeling subcell degradation rates in perovskite-silicon tandem solar modules

Source：pv magazine

To be able to identify the tolerable degradation rate of perovskite subcells in monolithic two-terminal tandem modules, researchers at Delft University of Technology (TU Delft) in the Netherlands developed a dual model. It predicts lifetime energy yield and degradation rates under a variety of environmental conditions.

“In this study, the novelty is that we combine a physical approach with a scenario-based approach in order to determine the tolerable degradation rate,” first author of the research, Youri Blom, told pv magazine, noting that the fully simulation-based approach relied on measurement data from several literature sources.

Similar studies in the past have relied on either a physical approach for conventional crystalline silicon modules or a fully scenario-based approach for perovskite-based modules.

“To see how the tolerable degradation rate of the perovskite cell depends on the climate, we combine these two approaches to have i) climate-dependent information for the silicon bottom cell, and ii) a scenario approach for the perovskite top cell,” explained Blom, noting that modelling is required for perovskite devices because there is insufficient outdoor data available to calibrate and design physical equations.

After identifying the tolerable degradation rates for the perovskite cell, ensuring that the perovskite-silicon module continues to outperform the crystalline silicon module, various degradation scenarios were analyzed. The degradation mechanisms included in the model were discoloration, moisture-induced degradation (MID), thermal cycling-induced degradation (TC), and light-induced degradation (LID), as these mechanisms are most severe or occurring most frequently, according to the paper.

Limitations of the previously validated PVMD Toolbox used for building the model did not permit to consider hotspot and potential induced degradation (PID), the researchers noted.

The environmental stress factor-dependent degradation was investigated four locations, each with a distinct climate: Delft, Lagos, Lisbon and Shanghai. The cell modelled was a 144 half-cut perovskite-silicon tandem device with a G12 wafer size connected in a butterfly topology with three bypass diodes. The cell stack was based on a 32.5% efficient two terminal design from the literature.

The results showed that in Delft, “where there is a high module lifetime, the perovskite top cell needs to be very stable as a degradation rate of only 1.9% can be tolerated,” but in Lagos, where the “module lifetime is short, a larger perovskite degradation rate of 7.6% can be tolerated,” according to the paper.

The team noted that the type of degradation, either current or voltage loss, in the subcell influences the overall power loss differently. For example, a 10% current loss in the perovskite subcell results in an 8.7% power loss, while a 10% voltage loss leads to a 6.2% power loss. “This shows that the degradation in both the perovskite and silicon subcell is relevant, and a single degradation rate is not sufficient,” it said.

Further results demonstrated that increasing module efficiency from 28.0% to 32.9% raised the tolerable degradation rate by approximately 50%.

As part of the research, the group developed a simplified model to reduce computationally intensive simulations, an Arrhenius-based model. Module efficiency and ambient temperature are used to “effectively and accurately” predict results for chosen locations with a root mean square error of 0.34% per year.

In addition, an efficient empirical model was created to calculate the degradation rate for other locations and cell efficiencies. “It can be used by other researchers or manufactures,” said Blom

The model may also be applied to environmental impact studies of perovskite-silicon modules. “By using life cycle analysis (LCA) results from literature, the comparison between crystalline silicon and perovskite/silicon can be extended to environmental footprint as well,” said Blom.

Currently, the research group is working on topics related to the circularity of PV modules, such as analyzing PV sustainability including PV-related critical raw material demand, designing PV modules for circularity as shown in its recently reported work on liquid PV module encapsulation, and investigating module aging, which includes this study, according to Blom.