Researchers at the US Department of Energy’s Lawrence Berkeley National Laboratory have created an inexpensive solar cell that responds to virtually the entire solar spectrum.
’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,’ said Wladek Walukiewicz, who leads the Solar Energy Materials Research Group in the Materials Sciences Division (MSD) at Berkeley.
One way to combine different band gaps is to stack layers of different semiconductors and wire them in series. This is the principle of current high-efficiency solar cells that use three different semiconductor alloys with different energy gaps.
However, even when the different layers are well matched, the structures are still complex — and so is the process of manufacturing them.
Another way to make a full-spectrum cell is to make a single alloy with more than one band gap. To manufacture such alloys is complex and time consuming, and the solar cells are also expensive to produce in quantity.
The new solar-cell material developed by Walukiewicz and Kin Man Yu and their colleagues in Berkeley Lab’s MSD and RoseStreet Labs Energy, working with Sumika Electronics Materials in Phoenix, Arizona, is a multiband semiconductor made from a highly mismatched alloy.
In this case, the alloy is gallium arsenide nitride, similar in composition to one of the most familiar semiconductors, gallium arsenide. By replacing some of the arsenic atoms with nitrogen, a third, intermediate energy band was created.
Finding the right combination of alloys, and determining the right doping levels to put an intermediate band right where it is needed, is mostly based on theory, and the new solar cell was developed using a band anticrossing model developed at Berkeley Lab over the past 10 years.
With the new, multiband photovoltaic device based on gallium arsenide nitride, the research team has demonstrated that a simple solar cell can respond to virtually the entire solar spectrum. Better still, it can readily be made using metalorganic chemical vapour deposition (MOCVD) — one of the semiconductor industry’s most common manufacturing techniques, so it should be inexpensive to produce.