Performance Modeling and Analysis of One-Year Outdoor Data of a 4-Terminal GaAs/Si Tandem Module

In this study, we analyze the long-term outdoor performance of a 4-terminal mechanically stacked GaAs/Si tandem mini module deployed for over one year in Golden, Colorado, USA. Current–voltage measurements of each sub-cell were taken every five minutes, with the other sub-cell operating at its maximum power point.

We employed a set of filter criteria to select measurement points proximate to standard test conditions (STC), facilitating the extraction of corresponding diode parameters for the sub-cells. Additionally, we determined the temperature coefficients of the sub-cells from outdoor data and applied a correction factor based on precipitable water vapor to account for temperature-correlated spectral changes impacting the GaAs top cell. Utilizing these correction coefficients, we can convert unfiltered data points to STC power ratings and extract raw power production data.

The translated performance data revealed a dimensionless performance ratio and degradation rates for each sub-cell, determined through linear regression analysis. The analysis showed degradation rates of 4.10% per year for the GaAs top cell and 2.48% per year for the Si bottom cell. The largest degradation was observed in the short-circuit current density (J_sc). We investigated our module packaging after outdoor deployment and found a delamination between the front glass and encapsulant, which is likely the cause for the performance degradation.

To confirm that package degradation is the main contributor to performance decline, we simulated the outdoor performance of the module and the individual sub-cells using PVcircuit, a circuit solver for tandem cells. PVcircuit was used with all parameters held constant except for the J_sc, ensuring that any changes in performance could be attributed to variations in J_sc. The simulation results showed excellent agreement with the measured module power, suggesting that the reduction in J_sc could explain the observed performance degradation. This finding is consistent with the identified delamination failure mode in the module packaging, known to primarily reduce the short-circuit current. Thus, the modeling results confirm that packaging degradation is a main contributor to the observed performance decline.

Our findings indicate that III-V-on-Si technologies are suitable for extended outdoor deployment, provided that appropriate module packaging is used. This type of analysis can be extended to emerging tandem technologies, such as perovskites-on-Si. At the conference we will show detailed results on our degradation analysis and the applied modeling approach for tandems.