Technical issues for the performance characterization of silicon-based thin-film multi-junction devices, such as the a-Si/thin-film c-Si structure, are discussed. The measured spectral response of each component cell is affected by the shape of the I-V curves of the component cells under color bias light, as well as the applied bias voltage. This paper describes procedures for correcting the measured spectral response. High-fidelity solar simulators, which incorporate Xe lamp(s) and halogen lamps that simulate AM1.5G standard sunlight, are very useful for characterizing the I-V curves of the multi-junction devices. They can accurately reproduce the current generated in each component cell and the I-V curve of the devices under standard sunlight, with a simple spectral adjustment procedure, thanks to their basic good spectral fidelity to standard sunlight. The irradiance dependence of the I-V curve is important for translating the I-V curve under different irradiance conditions. Although translation is not straightforward for multi-junction devices, the experimental results of the present study suggest that it is possible under spectral conditions where the relative value of the photocurrent among the component cells is constant. Determination of the I-V curve of each component cell based on the I-V curves of the multi-junction devices is also mentioned.