Measurement of Multi-junction (Perovskite tandem) Solar Cells

Perovskite solar cells have emerged as a promising candidate for tandem solar cells, which combine two or more photovoltaic materials to achieve higher efficiencies. The advantages of perovskite solar cells include their wide bandgap, which allows them to harvest more photons from the visible spectrum, and their solution processability, which enables low-cost and scalable fabrication methods. Perovskite solar cells also have the potential to be tuned by varying their composition, structure, and morphology.

Like other multi-junction solar cells, Perovskite Tandem solar cells have different spectral dependencies for each sub-cell, making the correction of spectral mismatch errors complex. This complexity makes accurate measurement (photovoltaic conversion characteristic evaluation) more technically challenging than for single-junction solar cells.

To evaluate the performance of PV devices under a certain reference spectrum, we use a solar simulator and a current-voltage (I–V) measurement system. However, the simulator spectrum may not exactly match the reference spectrum due to variations in the light source.

For single-junction devices, we can correct this spectral mismatch by adjusting the calibration of the reference cell under the simulator spectrum. This way, the test device will respond as if it were under the reference spectrum. The spectral mismatch correction factor indicates how much we need to adjust the total irradiance of the simulator to achieve this correction. The correction factor is determined by comparing the short-circuit current of the test device and the reference cell under both the simulator spectrum and the reference spectrum. The ratio of these currents equals the correction factor, which can be used to adjust the simulator's irradiance.

Multi-junction (Perovskite Tandem) Solar Cells present a more complex situation because each junction has its own reference cell that matches it, resulting in different spectral mismatch correction factors (Figure 1). This complexity makes correcting spectral mismatch errors challenging. To accurately measure their photoelectric conversion characteristics, a solar simulator that closely matches the reference spectrum is required. In other words, simply changing the total irradiance in the simulator is insufficient because each junction requires a different adjustment in total irradiance. Therefore, it is necessary to measure the characteristics under a solar simulator with a spectral irradiance spectrum that matches the reference spectrum as closely as possible.

To ensure high-accuracy measurements under standard test conditions (STC), spectrally adjustable solar simulators are essential. These simulators come in two main types: single- source, which often require time-consuming adjustments using optical filters, and multi- source, which offer independently adjustable spectrum ranges and are thus more efficient for testing Multi-junction devices. The adjustment process for spectral irradiance is iterative and varies depending on the specific simulator and spectroradiometer models used, aiming to achieve the target current matching factor for each cell junction. Consequently, multi- source simulators are preferred in PV testing laboratories due to their convenience and efficiency.

Citation: Song, T., Mack, C., Williams, R., Friedman, D. J., & Kopidakis, N. (2022). How Should Researchers Measure Perovskite-Based Monolithic Multijunction Solar Cells' Performance? A Calibration Lab's Perspective. Solar RRL, 2200800.