Solar efficiency: This diagram shows how the new composite material traps sunlight so efficiently.
Even though the microwire arrays are quite sparse, the reflective particles ensure that very little light escapes before it’s absorbed. The Caltech group has not yet published details of the material’s performance as part of a solar cell, but the composite has demonstrated very good numbers for light absorbance and electron carrier collection.
“There are three things a solar cell has to do: it has to absorb the light, collect all the [electrons], and generate power,” says Atwater. The material can absorb 85 percent of the sunlight that hits it, and 95 percent of the photons in this light will generate an electron. Until the results are published, the Caltech group won’t disclose their power generation results.
“What’s exciting is, you can use a lot less material to make a solar cell–two orders of magnitude less,” says Yi Cui, professor of materials science at Stanford University. This will do more than just lower the material’s costs. “Once you use less material for deposition, your manufacturing line is shorter,” Cui explains. This has two business implications: it should take less capital investment to build the factories needed to make the cells, and it should be possible to produce them at a faster rate.
Atwater’s group is now working on making the photovoltaic material over a larger area and incorporating it into prototype solar cells. The results published so far come from proof of concept experiments using square centimeters of the material. “We have to do the normal unglamorous engineering: making low-resistance electrical contacts, and making large areas, hundreds of square centimeters,” says Atwater. He adds that although the material is put together in a novel way, it can be made using a combination of techniques that are well established and scalable.
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