TECHNOLOGYPHOTOVOLTAICS
curves derived by EL-analysis and those yielded by a sun-simulator (see Figure 4). This figure illustrates the great strength of the EL-characterization technique: In addition to predicting the combined current-voltage characteristics of the multi-junction solar cell, it reveals, in detail, all sub-cell current- voltage characteristics, including fill-factors and absolute voltage properties.
In principle, it is possible to derive the current- voltage curves under any spectrum of interest with our approach. These curves are extracted by inserting values for the photocurrent density of each sub-cell, which can be found by integrating the measured quantum efficiency with the new spectral operation condition. Based on the resulting current-voltage curves it is possible to estimate the performance, efficiency and fill factor – both at the sub-cell and multi-junction level – under the desired spectral operation condition of interest (see Figure 5). Catering for resistance effects is easy, too.
Our development of the EL technique has shown that it has a great deal to offer to both developers and manufacturers of multi-junctions cells. It provides qualitative and semi-quantitative information on lateral and vertical inhomogeneities, making this approach an attractive technique for fast in-line inspection during the cell growth process and module assembly. In addition, our EL technique is unique in being able to yield detailed information concerning the current-voltage curves of individual sub-cells, which hold the key to precise device improvement. What’s more, fill- factor behaviour and the efficiency under arbitrary illumination conditions can be determined by our novel approach, aiding long-term energy harvesting analysis.
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Further reading M. A. Green et. al. Progress in Photovoltaics: Research and Applications 19 84 (2011) H. Yoon et. al. Progress in Photovoltaics: Research and Applications 13 133 (2005) M. Yamaguchi et. al. Solar Energy Materials and Solar Cells 90 3068 (2006) C. G. Zimmermann IEEE Electron Device Letters 30 825 (2009) C. G. Zimmermann Journal of Applied Physics 100 23714 (2006) S. Roensch et. al. “Subcell I-V-characteristic analysis of GaInP/GaInAs/Ge solar cells using electroluminescence measurements”, to be published. U. Rau et. al. Physical Review B 76 085303 (2007) T. Kirchartz et. al. Appl. Phys. Lett. 92 123502 (2008)
Figure 4: I-V characteristics with fill factor (FF) and efficiency (h) of the sub-cells and the triple- junction solar cell (3J) under AM0 illumination conditions. The plot includes also the directly measured I-V curve of the triple-junction solar cell (3J) by a sun simulator
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Figure 5: I-V characteristics with fill factor (FF) and efficiency (h) of the subcells and the triple- junction solar cell (3J) under illumination conditions with reduced illumination in the absorption range of the top cell. The plot includes also the directly measured I-V curve of the triple-junction cell (3J) by a sun simulator.
www.solar-pv-management.com Issue III 2011
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