Elucidating the Process of Accurate Spectral Calibration for Tandem I-V Measurement using Multi-Lamp Light Sources

Reproducing the reference spectrum, AM1.5G, using solar simulators is essential for reporting the I-V performance of tandem solar cells. The International Electrotechnical Commission (IEC) presents the classification of solar simulators spectra in IEC 60904-9 and the spectral calibration for tandem devices in IEC 60904-1-1. However, implementation of these standards using multi-lamp light sources can be challenging. This study presents a process for fulfilling both IEC standards using a multi-lamp light source with 21 tunable intensity channels of different peak wavelengths. A spectrum fitting method based on Gram-Schmidt orthonormalisation is introduced to approximate the AM1.5G spectrum from a known set of LED inputs, enabling forward modelling of the output spectrum. Non-linearity and spectral shift corrections ensure that the modelled spectrum remains accurate. A calibration process is presented to satisfy both solar simulator classification and the IEC-defined spectral mismatch factor and matching factor thresholds. The method is validated on three perovskite–silicon tandem cells, all of which achieved | 1-M | < 0.05 and |1-Z | < 0.03 and for both sub-cells. This framework enables standard-compliant spectral simulation of AM1.5G for I-V of perovskite-based tandem devices.