A Step Toward Inexpensive,
Efficient Solar Cells
Solar energy offers promise for environmental reasons and as a power source for remote locations. However, contamination by metals (which inevitably occurs during manufacturing) can lower silicon solar cells' efficiency so much that they are not economically viable. Scott McHugo, investigating this problem at Beamline 10.3.1, has characterized how copper and nickel contaminants cluster in silicon and observed their partial disappearance under thermal treatment.
The micrographs above are from the same small region of a polycrystalline silicon solar cell. This region was chosen for study because of its short diffusion length. (Diffusion length is the distance that charge carriers migrate within the wafer before recombining with opposite-charge particles. The presence of metal contaminants tends to shorten diffusion length, thus impairing the solar cell's efficiency.) The nickel and copper contaminants appear in the same places; scanning electron microscopy revealed that these places correspond to dislocations in the silicon crystal structure.
The second set of micrographs, above, was taken after annealing (heating) the solar cell for 30 seconds at 500 degrees Celsius. Most of the nickel and copper clusters are dissolved by this treatment, but contamination is clearly still present.
The next step was gettering: heating in the presence of a metal reservoir (in this case, an aluminum layer on the back of the cell), which draws the impurities out of the cell. The micrographs above were taken following gettering at 800 degrees Celsius for 3 hours. No nickel or copper contaminants are detectable above the noise level for the scans.
This research was conducted by Scott McHugo (Berkeley Lab), using the Center for X-Ray Optics x-ray fluorescence microprobe at Beamline 10.3.1. Sample provided by M. Imaizumi and M. Yamaguchi, Toyota Technological Institute (Japan).
Funding: Office of Basic Energy Sciences of the U.S. Department of Energy.
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