Impact of Irradiation-induced Filter Temperature Increase on Calibration of Reference Solar Cells with NIR-Longpass Filters

Reference solar cells are essential to the performance rating of photovoltaic devices. Si, KG-filtered Si, and GaAs reference cells are the common reference cell types used in the calibration chain of mainstream PV technologies. For emerging tandem PV technologies such as, GaAs/Si, CdTe/Si, and perovskite/Si, the spectral responses of their bottom junctions are only in near-infrared (NIR) region. Hence, these conventional reference cells are no longer suitable for their performance rating as their poor spectral matching can introduce large measurement errors. Si reference cells with colored glass NIR-longpass filters are preferrable as they have a much better spectral matching. However, this type of reference cells shows ~4-8 times larger Isc variation than other conventional reference cells in our primary calibration process. It is found that the cut-on wavelength of NIR-longpass filters (e.g., RG715 and 850 nm Schott filters) assembled on these reference cells is temperature-sensitive. In a 5-min calibration period, the longpass filters can be gradually heated up by over 5 °C due to the absorption of visible light and accordingly their cut-on wavelength increases by ~1-3 nm. As a result, their measured Isc gradually decreases by ~0.5-1.0%, causing additional large measurement uncertainty in the primary calibration. We also show that actively cooling the filters and/or choosing thinner filters are effective approaches to reduce the Isc variation to a ~0.2% level as they enable the NIR filters to reach a stabilized temperature faster.