Characterization of the Silicon-Based Thin Film Multi-Junction Solar Cells
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Characterization of the Silicon-Based Thin Film Multi-Junction Solar Cells Yoshihiro HISHIKAWA National Institute of Advanced Industrial Science and Technology (AIST) Research Center for Photovoltaics Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki, 305-8568 Japan ABSTRACT 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.
INTRODUCTION The I-V curves of multi-junction solar cells and modules (hereafter referred to as multi-junction devices) show more complicated features than those of single-junction devices, since multiple component cells with different characteristics are series-connected in one device[1-7]. In this study, procedures for accurately characterizing the multi-junction devices indoors are discussed, with special attention paid to the evaluation of the spectral responses of the component cells and the adjustment of the solar simulator spectrum. Dependence of the I-V curve on irradiance, and separation of the I-V curve of each component cell, which are useful for estimating their properties under different climatic conditions, are also mentioned. DISCUSSION Procedure for Characterizing Multi-Junction Devices Performance of multi-junction devices is strongly affected by the balance of the I-V curves of each component cell. In particular, the total output current tends to be limited by the minimum output current among the component cells, due to the requirement of current continuity among series-connected component cells. Therefore, during indoor I-V measurements using solar simulators, the spectral irradiance of the light which enters each component cells should be
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the same as that under the target spectrum such as the AM1.5G standard sunlight spectrum. If the spectral dependence of the I-
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