# The Outdoor Field Test and Energy Yield Model of the Four-Terminal on Si Tandem PV Module
## 5 Conclusions  
Tandem solar cells are highly efficient, and various types of the device structure were studied. On-Si tandem is one of them, and it has a distinct advantage of cost, using well-established Si solar cell technology. Regardless of the type of material, the annual performance of the tandem solar cells does not perform well due to the spectrum mismatching loss. The modeling of the spectrum mismatching loss was studied relying on the airmass variation. The intensive study on CPV performance in more than 20 years revealed that the fluctuation of atmospheric parameters played an essential role. Due to the development of the new application of the high-efficiency solar cell, including vehicle-integrated solar cells, the precise annual energy yield modeling of the tandem solar cells is required. The knowledge of precise spectrum-mismatching modeling in CPV is expanded to the non-concentration standard installation. 4T on Si tandem solar cell is a good candidate for robustness to spectrum variation. Its outdoor operation and energy yield modeling was intensively studied in this article. The model did not rely only on airmass but considered real fluctuation of the spectrum in all kinds of climates considering atmospheric fluctuation. Many previous studies tried to calculate the spectrum conditions only by airmass and also tried to apply the model that was developed for clear-sky conditions. The airmass-based calculation was proved inaccurate in the intensive investigation of CPV modules using three-junction tandem cells. The lack of an all-climate spectrum model was not the issue for the application to CPV, because it only used the direct sunlight. However, for the application to the standard installation (non-concentration), we must count the energy generation on the partially clear-sky days and cloudy days. A new idea of approximation of the all-climate spectrum model was developed and tried in this study, and it successfully modeled the energy yield of the tandem cells in all climates. The advantage of the 4T configuration of the photovoltaic module using a triple-junction solar cell, precisely, InGaP/GaAs on Si solar cell, was compared with a conventional 2T triple-junction module, specifically, InGaP/GaAs/InGaAs solar cell. The behavior of seasonal variation of performance and spectrum influence was modeled and validated by outdoor measurement. The annual amplitude of the seasonal peak-to-peak performance ratio improved by about 40%. This robust performance of the seasonal fluctuation of the solar spectrum is useful to applications to high-performance photovoltaic applications like zero-emission buildings and light-weight aerospace applications [108]. For the use in vehicle-integrated photovoltaic systems, it is also essential to consider 3D solar irradiance (modeling [109] and measurement [110]), as well as the performance of the photovoltaic unit on a curved surface (modeling [111; 112] and the standard [113]). The seasonal fluctuation of the 2-terminal triple-junction solar is also responsible for seasonal variation of the water precipitation. The above-validated model was expanded to various climate zones. The advantage of 4T on Si configuration increases (subarctic zone \(<\) temperate zone \(<\) subtropical zone). It is important to note that the model we developed and validated is not only applied to 4T InGaP/GaAs on Si solar cell, but can be extended to every type of the tandem solar cells in principle, including Perovskite on Si, Perovskite on CIGS, Polymer tandem, and III-V tandem, regardless of monolithic, wafer-bonding, and mechanical stack. It can even be applied to partial concentrator tandem cells [77] and super-multi-junction solar cells [1]. Conceptualization: Y.O., and K.A. ; methodology: H.T., H.S., Y.O., and K.A. ; validation: H.T., H.S., Y.O., and K.A. ; formal analysis: H.T., H.S., and Y.O. ; data curation, H.T., H.S., and Y.O. ; writing--original draft preparation, H.T., K.A. and Y.O. ; writing--review and editing: H.T., Y.O., and K.A. ; supervision: K.N. and M.Y.