Synchrotron X-ray microprobe contributes to development of cheaper solar cells

Professor E. Weber's team at Berkeley, California, US has recently succeeded in finding a new technique to handle metal defects in low-grade silicon, which could dramatically reduce the cost of solar cells. At present, around 90 % of solar cells in the world are made from a refined, highly purified form of silicon. This is because solar cells made from cheaper forms of silicon do not perform well and also because removing impurities is expensive. The new idea is to manipulate the impurities in a way that reduces their detrimental impact on the solar cell, instead of purifying the material. The team analyzed how metal contaminants in silicon respond to different types of processing using a synchrotron X-ray microprobe capable of detecting metal clusters as small as 30 nanometers. In addition to micro-XRF and micro-XAFS, they employed a new method based on a spectrally resolved X-ray-beam-induced current, which generates a map of the minority-carrier diffusion length, revealing the precise impacts of metal impurity clusters on local material performance. They found that they were able to manipulate the distribution of the metal impurities by varying the cooling rate of the silicon. When the material is cooled quickly, the metal defects are quickly locked in a scattered distribution. For more information, see the paper, "Engineering metal-impurity nanodefects for low-cost solar cells", T. Buonassisi et al., Nature Materials, 4, 676-679 (2005).