Spatially Resolved and Subcell-Selective Implied Open-Circuit Voltage Measurements on Perovskite Silicon Tandem Solar Cells

As the efficiency of perovskite silicon tandem solar cells continues to improve [1], the commercialization of such tandem solar cells is gaining attention. This development is closely tied to the trend of increasing the size of tandem solar cells. However, as the area increases, so does the challenge of maintaining uniformity across the cells. On non-uniform large-area solar cells, the spatial resolution of measurements for process control and characterization becomes critical. The implied open-circuit voltage iVoc is one of the key parameters to evaluate the quality of a solar cell. To measure iVoc spatially resolved, various methods such as micro-PL measurements, hyperspectral PL imaging, and calibrated lifetime images based on PL imaging have been described in literature [2-6]. However, the rasterizing micro-PL method is very time-consuming when applied to large areas, and hyperspectral imaging requires complex tunable filter systems. Additionally, PL-calibrated lifetime measurements on perovskite subcells are challenging due to the short lifetimes compared to silicon. In contrast, spectrally integrated absolute PL imaging methods [7, 8] can overcome these shortcomings. We present a fast and contactless method, inspired by [7, 8], for extracting spatially resolved iVoc images from spectrally integrated absolute PL images. The PL signal is measured with a silicon CCD camera on areas up to the M12 wafer format and calibrated to an absolute photon flux using a halogen calibration lamp. This method is applied to perovskite silicon tandem solar cells for the first time, using two lasers with different wavelengths for the selective excitation of each subcell. In contrast to the method described in [7], we use wide-bandpass filters. This allows to apply the method to a wider range of bandgaps for a given set-up. Using simulated absorption spectra instead of 1−R spectra, we do not need to determine the quantitative absorptivity of the top solar cell. Hence, we can use bandpass filters that are sensible to a larger part of the PL spectrum and overcome the problem of long integration times due to low photon fluxes in the high-energy tail of the PL spectrum. The measurement results are validated by spectrally resolved, absolute PL measurements with an integrating sphere and PL-calibrated lifetime images.