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Gold may not seem like the best material to use to reduce costs, but scientists think it could help produce the most affordable solar cells yet. Stanford University researchers announced Thursday that they used minute globs of gold to create the thinnest light absorbers ever. Solar cells made with the gold nanodots would require much less material, potentially making them cheaper.
The researchers coated wafers with 520 billion gold nanodots per square inch in a honeycomb-like pattern. The dots measured 14 nanometers tall and 17 nanometers wide — thousands of times thinner than a sheet of paper, though still thicker than graphene. They coated different sets of nanodots with tin sulfide, zinc oxide and aluminum oxide to test their properties.
The wafer, dots and coating combined to create a light absorber 1,000 times thinner than commercially available options. The previous record was three times thicker.
“It’s a very attractive technique, because you can coat the particles uniformly and control the thickness of the film down to the atomic level,” study lead Carl Hagglund said in a release. “That allowed us to tune the system simply by changing the thickness of the coating around the dots.”
They modified the thickness so that the wafer and nanodots would best absorb reddish-orange light. The wafers absorbed 99 percent of the light, while the dots absorbed 93 percent.
“Much like a guitar string, which has a resonance frequency that changes when you tune it, metal particles have a resonance frequency that can be fine-tuned to absorb a particular wavelength of light,” Hagglund said. “We tuned the optical properties of our system to maximize the light absorption.”
Many materials are capable of absorbing 90 percent or more of certain light wavelengths, or colors. When they are incorporated into a solar cell, the total efficiency drops. The best commercial cells out there right now hit about 15 to 20 percent efficiency. Better options could be on their way.
Ideally, a solar cell would be able to absorb all visible light, instead of just certain wavelengths. The researchers have not shown how that can be done with gold nanodots, but they plan to go ahead and demonstrate that they work in actual solar cells. They also plan to look into different nanodot materials, including silver, which is less expensive and a better absorber. They are using gold for now because it is more stable.