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Catching More Rays


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A new semiconductor may lead to improved solar cells that capture a wider range of sunlight’s energy–and pump out more electricity.

When sunlight strikes a semiconductor solar cell, energy from the photons knocks loose electrons in the semiconductor, allowing current to flow. But only particular bands of wavelengths have just the right amount of energy to do the trick. Existing silicon solar cells, which have just one such band gap, turn about 15% of light energy into electricity. Combining materials with various band gaps is one way to boost their efficiency. Layering up to three materials can improve the efficiency to about 30%, but that’s difficult and expensive to do, limiting the resulting solar cells to high-tech applications such as satellites. Researchers have theorized that a single semiconductor with multiple band gaps would be more efficient, but so far they haven’t been able to manufacture one.

Now a group has engineered an alloy of zinc, manganese, and tellurium, with a touch of oxygen, that has two band gaps. One wavelength of light sends electrons into an excited state; another wavelength knocks resting electrons into a second, higher energy excited state. As a bonus, yet a third wavelength can knock electrons from the first excited state to the second, potentially converting even more of the light energy into electricity. In a paper to be published in Physical Review Letters, Kin Man Yu of Lawrence Berkeley National Laboratory and colleagues calculate that in a solar cell, the material would be 45% efficient. And tuning the oxygen content could boost the efficiency above 50%, they suggest.

The concept “is very intriguing,” says Paul Sharps, development manager at Emcore Photovoltaics, a solar cell manufacturer in Albuquerque, New Mexico. However, he cautions, “there are many, many issues that have to be addressed before this material can ever be considered for use as a solar cell.” For example, he says, charges have trouble moving through semiconductors made of alloys, which could make it hard to extract power from the new material. Nevertheless, the authors have taken an important step by moving from theory to an actual material, says Antonio Marti of the Polytechnic University of Madrid.