Affordable, full spectrum solar cells

National researchers design a new solar device that absorbs infrared and ultraviolet light, and all the colors in between. At the end of this rainbow could be cheaper, more efficient solar power.

From infrared to ultraviolet, a new solar cell can respond to the wavelengths of almost the entire light spectrum. And making it might be relatively easy.

So say researchers, published this week in Physical Review Letters, who have designed this new multi-banded solar cell. The cell could help make solar devices more efficient, because more of the sun's light would be available for conduction.

Current high-efficiency solar cells typically consist of three semiconductor layers with varying band gaps, or energy gaps. The band gap is what determines which wavelengths are best absorbed by the semiconductor.

“Since no one material is sensitive to all wavelengths, the underlying principle of a successful full-spectrum solar cell is to combine different semiconductors with different energy gaps,” says lead researcher Wladek Walukiewicz in a statement.

Full spectrum solar cells are not new. The scientists—from Lawrence Berkeley National Laboratory and Rose Street Labs Energy—produced two others over the last decade. These solar cells each had varying mixes of one alloy, which basically served as different semiconductors. The first used indium gallium nitride and the second had zinc, manganese and tellurium. According to their developers, both would have been prohibitively pricey for commercial purposes, with manufacturing being complex and time-consuming. Their latest creation, however, is simpler.

This time their alloy is gallium arsenide nitride (similar to gallium arsenide, which is often used in thin-film solar cells). They produced the alloy through metalorganic chemical vapor deposition, a process also commonly used by the industry.

Adding an intermediate band between the conduction and valence bands was key (see image above). Although the band does not conduct a charge, it acts as "a stepping stone" for currents of the medium energies that the other two bands aren't picking up. The researchers created the intermediate band by switching some arsenic molecules with nitrogen. With three band gaps that cover the light spectrum, the device was able to absorb energies between 1.1 electron volts (eV) in the infrared range to 3.2 eV at the ultraviolet end of the spectrum.

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