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In recent weeks, some of the most powerful solar flares ever witnessed have sent electrically charged gas shooting toward the earth in so-called coronal mass ejections. But even without such impressive displays, the sun provides a wealth of energy to our planet. Unfortunately, efforts to mass produce solar cells to harness energy from Sol have stalled at efficiencies of around 30 percent in the laboratory and less than 20 percent in commercial cells. A novel crystal described in a paper set to be published in the journal Physical Review Letters may change that, however. Scientists report that the semiconductor material could form the basis of solar cells with nearly 50 percent efficiency.
In a standard photovoltaic cell, the sun's rays are converted into electricity when electrons within the material are knocked loose. To accomplish this the incoming light must have a specific energy, known as the band gap. Incident light with less energy will not be absorbed, while the portion of more energetic radiation above the band gap will be lost. In an attempt to alleviate this problem, Kin Yu of Lawrence Berkeley National Laboratory and colleagues investigated the properties of a new semiconductor material comprising an alloy of zinc, manganese and tellurium.
The researchers added oxygen impurities to the crystal, which resulted in a crystal having three band gaps instead of the customary one. "These three absorption edges span much of the solar spectrum," they write, "thus these alloys are good candidates for the multi-band semiconductors envisioned for high efficiency photovoltaic devices." The scientists further theorize that the efficiency could be increased to as much as 56 percent by changing the ratio of the atoms or replacing manganese with magnesium.