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.<br/><br/>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.<br/><br/>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 (<i>J</i>_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.<br/><br/>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 <i>J</i>_sc, ensuring that any changes in performance could be attributed to variations in <i>J</i>_sc. The simulation results showed excellent agreement with the measured module power, suggesting that the reduction in <i>J</i>_sccould 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.<br/><br/>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.