Scientists closer to practical full-spectrum solar cells
By Ben Coxworth
January 31, 2011
Scientists from the U.S. Department of Energy’s Lawrence Berkeley National Laboratory have come a step closer to the development of a commercially-viable full-spectrum solar cell. Traditionally, due to their limited band gap (energy range), semiconductors used in solar cells have only been able to respond to a certain segment of the solar spectrum – this segment varies, according to the semiconductor. Some cells have been created that respond to everything from low-energy infrared through visible light to high-energy ultraviolet, but these have been costly to produce and thus unfit for common use. The new cell, however, responds to almost the entire spectrum, and can be made using one of the semiconductor industry’s most common manufacturing processes.
Given that no one semiconductor alloy can respond to all wavelengths, the approach used in the past has been to stack layers of different semiconductors – each one with a different band gap – and wire them in series. Nine years ago, by adjusting the amounts of indium and gallium in the alloy indium gallium nitride, Berkeley’s Wladek Walukiewicz and Kin Man Yu were able to tweak its band gap to respond to different wavelengths. Using this technology, they were able to create a full-spectrum solar cell by stacking different versions of the same alloy, but the production process was quite complex.
In 2004 they took a different approach, creating a single alloy of highly mismatched semiconductors based on a common alloy, zinc (plus manganese) and tellurium. They were able to add a third band gap, between those of the zinc and tellurium, by doping the alloy with oxygen. This once again resulted in a full-spectrum solar cell, but the method of creating it was once again too complicated and expensive.
Their latest creation is another multiband semiconductor alloy, gallium arsenide nitride, which has a composition similar to that of the commonly-used gallium arsenide. In this case, the third band is created by replacing some of the arsenic atoms with nitrogen. Unlike their previous efforts, this solar cell material can be produced via one of the most common methods of fabricating compound semiconductors – metalorganic chemical vapor deposition.
When exposed to sunlight, a test cell made with the new semiconductor was shown to respond strongly to all parts of the spectrum, making this a significant step towards more efficient solar cells that can be mass produced by conventional methods.
The research was recently published in the journal Physical Review Letters.
Via Berkeley Lab.